…rkError` and `BackendError` (#1425)

Closes #695 and finishes [CF-195]. 
Depends on ~~#1431, #1432, #1433, #1437~~.

## Goals:

- [x] Handle all requests / response / deserialization errors in `HTTPClient`. Clients of `HTTPClient` will only have to handle the "happy" path: #1431
- [x] `HTTPClient` shouldn't return `Result.success` with failed responses, forcing clients to verify the response is actually a failure: #1431
- [x] Abstract error response deserialization / error creation: #1427
- [x] Abstract attribute error parsing: #1427 
- [x] Move response deserialization to `HTTPClient` based on a `ResponseType: Decodable` type, so the completion block will simply return a `Result<HTTPResponse<ResponseType>, Error>`
- [x] `ETagManager` should store response `Data` instead of a deserialized `[String: Any]`
- [x] Improved error types in `HTTPClient` to fix tests: #1437
- [x] Dealt with non-backwards compatible `ETagManager: #1438

## Changes:
- Replaced `HTTPResponse`'s body from `[String: Any]` to a generic `HTTPResponseBody`.
- Created `HTTPResponseBody` to abstract `Decodable` and provide some default implementations for types like `Data,` `[String: Any]` (for backwards compatibility to types that aren't `Decodable` yet), and `Decodable` itself.
- New `HTTPResponse.Result` typealias (`Result<HTTPResponse<HTTPResponseBody>, Error>`) used everywhere. This will allow replacing `Error` with a more specific `Error` so we can forward known typed errors, and make sure that we don't end up with the wrong error type, or with a very complex error hierarchy and the details buried in `underlyingError`.
- Each layer (only a few for now) has its own error type: `NetworkError`, `BackendError`, `OfferingsManager.Error`.
- `HTTPClient` for example has to produce `NetworkError`, operations produce `BackendError`
- The parent `BackendError` can have specific errors like `.missingAppUserID`, but also be simply a child error `case networkError(NetworkError)`
- All of these conform to a `ErrorCodeConvertible`, so there is a single point of code that converts from simple and readable errors (like `BackendError.emptySubscriberAttributes`, `.unexpectedBackendResponse(.loginResponseDecoding)`) into errors with all the context using `ErrorUtils`
- Converted `DNSError` into `NetworkError.dnsError`. Its functionality remains unchanged.
- Removed `Backend.RCSuccessfullySyncedKey` and `ErrorDetails.finishableKey` in favor of tested properties on `NetworkError`

### Leftovers

There's a few things I've decided to not finish for now:
- [ ] `CustomerInfo` still does't conform to `Decodable` (manual deserialization is still supported by the current system): #1496
- `SubscriberAttributeDict` could also be `Codable`
- [x] Replace `OfferingsFactory` with `Decodable`: #1435

[CF-195]: https://revenuecats.atlassian.net/browse/CF-195?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

…rrorCodeConvertible` and added tests

Follow up to #1437.
Fixes [CF-323]

…rrorCodeConvertible` and added tests (#1473)

Follow up to #1437.
Fixes [CF-323]

… returned error types

This is the last change from what was started with #1437. That PR introduced a few private error types, but for the errors returned in the public `APIs` we were still passing around `Error`s everywhere, which didn't provide any compile-time guarantees.

- Ensure that public APIs *only* return `ErrorCode`s wrapped with the additional `userInfo`, and not accidentally return other types like `SKError`, `BackendError`, `ErrorCode` directly, etc.
- Guarantee type-safety when passing errors around within our internal implementations instead of assuming certain thrown errors are of a particular type.

- Created `PurchasesError` for wrapping an `ErrorCode` with the additional `userInfo`.
- Changed returned errors from `Error` to `NSError`. This is a change in the API, but the new type is a more specific `Error` so it shouldn't lead to any changes in client apps.
- Because `PurchasesError` isn't implicitly convertible to `NSError`, this ensures that we don't return those values directly, and instead use `PurchasesError.asPublicError`.
- Changed `ErrorCodeConvertible` to a now more precise `PurchasesErrorConvertible`.

Thanks to this new type-safety this PR fixes at least 2 bugs where we were returning private errors instead of `ErrorCode`s.

Thanks to #1871 we know that the returned errors are still convertible to `ErrorCode` so users can `switch` over them.

… returned error types (#1879)

This is the last change from what was started with #1437. That PR introduced a few private error types, but for the errors returned in the public `APIs` we were still passing around `Error`s everywhere, which didn't provide any compile-time guarantees.

### Goals:
- Ensure that public APIs *only* return `ErrorCode`s wrapped with the additional `userInfo`, and not accidentally return other types like `SKError`, `BackendError`, `ErrorCode` directly, etc.
- Guarantee type-safety when passing errors around within our internal implementations instead of assuming certain thrown errors are of a particular type.

### Changes:
- Created `PurchasesError` for wrapping an `ErrorCode` with the additional `userInfo`.
- Changed returned errors from `Error` to `NSError`. This is a change in the API, but the new type is a more specific `Error` so it shouldn't lead to any changes in client apps.
- Because `PurchasesError` isn't implicitly convertible to `NSError`, this ensures that we don't return those values directly, and instead use `PurchasesError.asPublicError`.
- Changed `ErrorCodeConvertible` to a now more precise `PurchasesErrorConvertible`.

Thanks to this new type-safety this PR fixes at least 3 bugs where we were returning private errors instead of `ErrorCode`s.

### Testing:

Thanks to #1871 we know that the returned errors are still convertible to `ErrorCode` so users can `switch` over them.