Partially addresses #4486.

### Context
CometBFT will stop importing
[cometbft-db](https://github.com/cometbft/cometbft-db) as a dependency
to support multiple database backends. Instead, it will use
[pebble](https://github.com/cockroachdb/pebble) by default.

### Changes
This PR adds package `storage` to the `internal` packages: 
- it exports the `DB`, `Batch`, and `Iterator` interfaces that database
backends must implement to work with CometBFT. Previously, they were
offered by cometbft-db.
- it exports the pebbledb implementation of the above interfaces.

Users can provide their own implementation of these interfaces using a
different database under the hood. However, we will support and maintain
only the pebble implementation that CometBFT will use.

**Note**: this PR ports the code currently provided by cometbft-db. It
does not add new functionalities to either the interfaces or their
implementation using pebble. What it does:
- improve docs
- clearer error messages
- minor refactoring of nested `if` statements for readability

---

#### PR checklist

- ~[] Tests written/updated~
- ~[ ] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)~
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments

…face. (#4515)

Partially addresses #4486.

### Context
CometBFT will stop importing
[cometbft-db](https://github.com/cometbft/cometbft-db) as a dependency
to support multiple database backends. Instead, it will use
[pebble](https://github.com/cockroachdb/pebble) by default.

### Changes
This PR adds unit tests to the pebble implementation of the `DB`
interface that CometBFT will use once we remove cometbft-db.

…terface. (#4522)

Partially addresses #4486.

### Context
CometBFT will stop importing
[cometbft-db](https://github.com/cometbft/cometbft-db) as a dependency
to support multiple database backends. Instead, it will use
[pebble](https://github.com/cockroachdb/pebble) by default.

### Changes
This PR adds unit tests to the pebble implementation of the `Batch`
interface that CometBFT will use once we remove cometbft-db.

… interface. (#4530)

Partially addresses #4486.

### Context
CometBFT will stop importing
[cometbft-db](https://github.com/cometbft/cometbft-db) as a dependency
to support multiple database backends. Instead, it will use
[pebble](https://github.com/cockroachdb/pebble) by default.

### Changes
This PR adds unit tests to the pebble implementation of the `Iterator`
interface that CometBFT will use once we remove cometbft-db.

…to_feature_remove-cometbftdb

Partially addresses #4486.

### Context
CometBFT will stop importing
[cometbft-db](https://github.com/cometbft/cometbft-db) as a dependency
to support multiple database backends. Instead, it will use
[pebble](https://github.com/cockroachdb/pebble) by default.

### Changes
This PR adds type `PrefixDB` to `internal/storage` from cometbft-db. 
`PrefixDB` is a database wrapper that implements the`DB`, `Batch`, and
`Iterator` interfaces.

**Note**: this PR makes some changes to the implementation of
`PrefixDB`. Namely:
- `PrefixDB` no longer uses a mutex, because we think that it is
sufficient to rely on the wrapped database's concurrency control
mechanism. That is, concurrent calls to `PrefixDB`'s methods will
already "lock" on the calls to the wrapped DB methods.
- `NewPrefixDB` now returns an error if the given prefix is empty. We
think allowing the creation of a `PrefixDB` with no prefix was an
oversight in the design.
- `prefixDBBatch` methods now want a pointer receiver. 
- `PrefixDB.Compact` now prepends the prefix to `start` and `end` keys
defining the iterator's range. The previous implementation did not do
so, and this was likely an oversight. We believed this behavior
contradicted the definition of a `PrefixDB` as a logical database that
prepends the prefix to all keys before operating on the underlying
database.

Partially addresses #4486.

This PR removes cometbft-db from cometbft build process:
- Dockerfiles
- Makefiles
- CI