…nueCat#998)

Fixes RevenueCat#988.

Unfortunately `ISO8601DateFormatter` inherits from `Formatter`, and not `DateFormatter`, so this introduces a new protocol `DateFormatterType` that both types can implement.

Another unfortunate difference with the new type compared to the old implementation is that milliseconds are either optional or mandatory.
For that reason, I added `ISO8601DateFormatter.default` which replicates the same behavior by deferring `date(from: String) -> Date` to one formatter or the other.
I added tests to cover this behavior directly, but it was only thanks to the existing `ArraySlice` tests that I became aware of this requirement.

Additionally, I made `Transaction.init(with:productId:dateFormatter)` not `public` and not `@objc` so it can take a new `DateFormatterType`.
The method didn't need to be `public`, it was just a pre-Swift-migration leftover.

As a bonus I changed some of the tests in `DateFormatter+ExtensionsTests` to use `XCTUnwrap`.

Fixes #988.

Unfortunately `ISO8601DateFormatter` inherits from `Formatter`, and not `DateFormatter`, so this introduces a new protocol `DateFormatterType` that both types can implement.

Another unfortunate difference with the new type compared to the old implementation is that milliseconds are either optional or mandatory.
For that reason, I added `ISO8601DateFormatter.default` which replicates the same behavior by deferring `date(from: String) -> Date` to one formatter or the other.
I added tests to cover this behavior directly, but it was only thanks to the existing `ArraySlice` tests that I became aware of this requirement.

Additionally, I made `Transaction.init(with:productId:dateFormatter)` not `public` and not `@objc` so it can take a new `DateFormatterType`.
The method didn't need to be `public`, it was just a pre-Swift-migration leftover.

As a bonus I changed some of the tests in `DateFormatter+ExtensionsTests` to use `XCTUnwrap`.

Fixes [TRIAGE-172]

See stacktrace:
```
Fatal Exception: NSInternalInconsistencyException
Invalid parameter not satisfying: formatOptions == 0 || !(formatOptions & ~(NSISO8601DateFormatWithYear | NSISO8601DateFormatWithMonth | NSISO8601DateFormatWithWeekOfYear | NSISO8601DateFormatWithDay | NSISO8601DateFormatWithTime | NSISO8601DateFormatWithTimeZone | NSISO8601DateFormatWithSpaceBetweenDateAndTime | NSISO8601DateFormatWithDashSeparatorInDate | NSISO8601DateFormatWithColonSeparatorInTime | NSISO8601DateFormatWithColonSeparatorInTimeZone | NSISO8601DateFormatWithFullDate | NSISO8601DateFormatWithFullTime | NSISO8601DateFormatWithInternetDateTime))
Fatal Exception: NSInternalInconsistencyException
0 CoreFoundation 0x141d04 __exceptionPreprocess
1 libobjc.A.dylib 0x8528 objc_exception_throw
2 CoreFoundation 0x141bd8 +[NSException raise:format:]
3 Foundation 0xacc24 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:]
4 Foundation 0xe4794 -[NSISO8601DateFormatter setFormatOptions:]
5 RevenueCat 0x4795c one-time initialization function for withMilliseconds
6 libdispatch.dylib 0x1048 _dispatch_client_callout
7 libdispatch.dylib 0x4710 dispatch_once_f$VARIANT$mp
8 RevenueCat 0x47b0c date(from:) in Formatter #1 in closure #1 in variable initialization expression of static NSISO8601DateFormatter.default + 60 (DateFormatter+Extensions.swift:60)
9 RevenueCat 0x47b68 protocol witness for DateFormatterType.date(from:) in conformance Formatter #1 in closure #1 in variable initialization expression of static NSISO8601DateFormatter.default (<compiler-generated>)
10 RevenueCat 0xe6b8 AppleReceiptBuilder.build(fromContainer:) (AppleReceiptBuilder.swift)
```

This is the key part:
`formatOptions == 0 || !(formatOptions & ~(NSISO8601DateFormatWithYear |
NSISO8601DateFormatWithMonth | NSISO8601DateFormatWithWeekOfYear |
NSISO8601DateFormatWithDay | NSISO8601DateFormatWithTime |
NSISO8601DateFormatWithTimeZone |
NSISO8601DateFormatWithSpaceBetweenDateAndTime |
NSISO8601DateFormatWithDashSeparatorInDate |
NSISO8601DateFormatWithColonSeparatorInTime |
NSISO8601DateFormatWithColonSeparatorInTimeZone |
NSISO8601DateFormatWithFullDate | NSISO8601DateFormatWithFullTime |
NSISO8601DateFormatWithInternetDateTime))`

This is coming from the following code:
```swift
static let withMilliseconds: DateFormatterType = {
    let formatter = ISO8601DateFormatter()
    formatter.formatOptions = [
        .withInternetDateTime,
        .withFractionalSeconds
    ]

    return formatter
}()
```

Even though `.withFractionalSeconds` is available in iOS 11, that
assertion seems wrong, as it's treating that option as invalid. This was
a regression in #998, almost a year ago, but since we have very little
coverage on iOS 11 we never noticed until now.

Unfortunately it seems like we won't be able to run iOS 11 tests in CI
(#2036), but this change is fairly safe, it just reverts to using the
old `DateFormatter` for older versions.

[TRIAGE-172]:
https://revenuecats.atlassian.net/browse/TRIAGE-172?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ