[simolus3]

This implements the suggested API simplification discussed here for Dart. Instead of passing a PowerSyncBackendConnector, users can implement Authenticator and MutationUploader interfaces separately. For MutationUploader, a default constructor can also wrap simple callbacks.

Implementing this without breaking existing users requires a bit of creative API design: Dart doesn't have method overloading, so we can't simply add another connect() method taking authenticators and uploaders separately. Instead, this replaces the PowerSyncBackendConnector parameter with ConnectRequest, a new supertype. With dot shorthands, I think this looks okay:

await db.connect(connector: MyBackendConnector()); // continues to work

// With split types
await db.connect(connector: ConnectRequest(
  endpoint: 'https://<...>.powersync.com/',
  authenticator: MyAuthenticator(),
  uploader: MutationUploader((db) async { /* ... */ })
))

// Download only
await db.connect(connector: .downloadOnly(
  endpoint: 'https://<...>.powersync.com/',
  authenticator: MyAuthenticator(),
))

// Upload only
await db.connect(connector: .uploadOnly(MutationUploader((db) async {
  // ...
})))

Splitting the interface makes it easier to get started with PowerSync (one can start with a download-only use case and worry about uploads later), and allows more flexible compositions (e.g. with a default write API uploader and a custom authenticator).

The changes to the streaming sync client are comparable to our other SDKs. Most of this PR deals with the annoying problem that connector methods are also invoked over RPC channels for isolates and web workers. This refactors the InternalConnector interface to split the authenticator / uploader parts as well. In a connect method, we then use two booleans to indicate whether each half is present.

TODOs: A follow-up item is that the new interfaces should also receive abort signals.

AI use: Reviewed with Claude Code.