Support bucket priorities and partial checkpoints

[simolus3]

Add support for partial checkpoint complete messages, which are forwarded to the core extension by providing the names of completed buckets within that partial checkpoint.
This also adds extends the waitForFirstSync and hasSynced APIs to be aware of priorities.

These changes are not backwards-compatible with old versions of the extension (which would always report an inconsistent partial checkpoint), but the extension changes are backwards-compatible with other SDKs that don't support partial checkpoints.

[rkistner]

These changes are not backwards-compatible with old versions of the extension (which would always report an inconsistent partial checkpoint), but the extension changes are backwards-compatible with other SDKs that don't support partial checkpoints.

Could you update the version check here when the new version is released?

powersync.dart/packages/powersync_core/lib/src/database/powersync_db_mixin.dart

Line 84 in 1f9268f

Future<void> _checkVersion() async {