Implement connection pool in Swift, remove Kotlin framework

[simolus3]

This implements SqlCursor and connection pools directly in Swift instead of depending on a Kotlin XCFramework. On a high level, this PR:

• Moves some protocol methods into extensions. For instance, the get$Type(name:) functions on SqlCursor are now implemented by delegating to get$Type(index:).
• Implement SqlCursor and related methods using CSQLite instead of using the Kotlin framework.
• Adds a Swift implementation of SQLiteConnectionPoolProtocol responsible for opening connections, configuring them and to dispatch queries on a background thread. The actual implementation is close to what we have on JavaScript, with an AsyncSemaphore helper guarding connections.
• Uses a direct function call to load the core extension! We can let the OS loader take care of loading the core extension framework, avoiding sqlite3_load_extension and having to resolve the path manually.

For the native SQLite wrapper, the implementation consists of these structs:

• NativeSqliteStatement is an owning (~Copyable) struct managing a SQLite statement and exposing raw SQLite methods to step through it.
• StatementCursor implements the existing SqlCursor protocol with a dynamic reference to a NativeSqliteStatement. Ideally, I would have preferred this to be ~Copyable as well and change APIs to give users a borrowing SqlCursor in callbacks for safety. That would be breaking though, so we instead have manual checks to verify a cursor isn't used outside of its callback.
• NativeConnectionPool manages a write connection (and an optional collection of readers). These are implemented through the owning RawSqliteConnection struct and an AsyncSemaphore with a design taken from our JS SDK implementation. We use the core extension with powersync_update_hooks to dispatch update notifications. For simplicity, this class is not responsible for opening connections or dispatching writes to a background thread.
• AsyncConnectionPool is responsible for opening connections and supports both in-memory databases and files in the default database directory (the design is so that we can potentially support custom paths in the future as well). It wraps a NativeConnectionPool for concurrency control and invokes callbacks on DispatchQueue.global(qos:)` to avoid blocking the main thread.
• PowerSyncDatabaseImpl implements high-level PowerSync APIs by opening a pool and running the usual initial statements to resolve the initial sync state.

Most of this is covered by existing tests, I've added a simple additional test covering how we bind parameter values.

[mintlify]

Docs PR opened: powersync-ja/powersync-docs#419

Opened a PR adding a docs-update tracker flagging that the Swift SDK is now pure Swift (no Kotlin/SKIE), so the client-sdks/reference/swift.mdx "Kotlin -> Swift SDK" section and CRUD-type samples need updating.

[stevensJourney]

This looks very nice and clean. I'm happy with these changes.

We should make a note to also check our docs which reference the Kotlin SDK e.g. https://docs.powersync.com/client-sdks/reference/swift#kotlin-%3E-swift-sdk . It looks like there was an autogenerated PR for this, but that was closed.

[simolus3]

We should make a note to also check our docs which reference the Kotlin SDK

Good catch, I'll prepare releases and will update the docs for that.