Add sync implementation from core extension

[simolus3]

This integrates the new sync logic implemented in the core extension (at powersync-ja/powersync-sqlite-core#70) into the Kotlin SDK.

Internally, the existing implementation has been renamed to "legacy implementation" (and I've annotated methods related to it with @LegacySyncImplementation which should make it easier to identify which methods we can eventually remove after we finish the migration).
For now however, the new implementation is marked as experimental, and not enabled by default. Users will have to use Kotlin's opt-in mechanism for ExperimentalPowerSyncAPI to use it. So, the only API changes are:

• Adding the SyncOptions class. Constructing a non-default instance of this class requires opting in to ExperimentalPowerSyncAPI.
  • On that class, a newClientImplementation field can be set to true to use the new Rust client.
  • Similarly, method can be set to ConnectionMethod.Http or to ConnectionMethod.WebSocket.

In the sync stream, I've moved the existing methods into an inner LegacyIteration class, the new ActiveIteration class implements the sync logic by forwarding it to the PowerSync control interface in the core extension. I've tested the new sync logic with websockets manually (although it might be worth setting up an internal RSocket server for that in the future). All sync tests are now running against both client implementations.