Database adapters: Share common logic

[simolus3]

A lof of our DBAdapter implementations are very similar:

• In all cases, the top-level get(), getOptional() and getAll() implementations delegate to readLock((ctx) => ctx.respectiveMethod(...)).
• In all cases, transactions are implemented with an internalTransaction method on readLock and writeLock (except for RNQS, where the exact same transaction wrapper is instead defined in @journeyapps/react-native-quick-sqlite).
• In all cases except Capacitor, get(), getOptional() and getAll() are implemented as wrappers over execute().

Given that the implementations are so similar, we should try to share them! Currently, RNQS, OP-SQLite, Capacitor, web, sql.js and node all use duplicate implementations that are exactly the same. If we add a Tauri SDK in the future, it would also have to duplicate this. This is also annoying for users: If an DBAdapter implementation didn't require around 100 lines of boilerplate, it would be much easier for users to write their own implementations (see e.g. #845 where that's relevant).

So, this:

1. Adds the ConnectionPool interface which just defines the "intrinsic" top-level methods (name, close, readLock, writeLock and refreshSchema). It adds the DBAdapterDefaultMixin mixin which can turn a class implementing ConnectionPool into a class implementing DBAdapter by delegating to readLock or writeLock, respectively.
2. Adds the SqlExecutor interface defining the execute, executeBatch and executeRaw methods. Similarly to the connection pool setup, DBGetUtilsDefaultMixin allows implementing get(), getAll and getOptional by wrapping execute.
3. Uses those mixins to remove duplicate code from the RNQS, OP-SQLite, Capacitor, sql.js and Node implementations.

The web worker setup is left unchanged for now, I'll revisit that in a follow-up PR. The reason is that I believe the web worker setup could benefit from adopting an architecture similar to the one used in the Node SDK, where clients contain most of the logic and workers just execute raw statements. But that's a non-trivial refactoring, so I wanted to get this out first.

I'm relying on existing unit tests for web, sql.js, node and op-sqlite here. I've tested capacitor manually, unfortunately I can't seem to get any RN demos to even compile on my machine.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8e1def5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@powersync/react-native	Minor
@powersync/common	Minor
@powersync/web	Minor
@powersync/op-sqlite	Patch
@powersync/capacitor	Patch
@powersync/node	Patch
@powersync/adapter-sql-js	Patch
@powersync/tanstack-react-query	Patch
@powersync/diagnostics-app	Patch

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[stevensJourney]

I really really love these changes. These are a welcome addition. Thank you for implementing this.

I tested the React Native Supabase Todolist (iOS) on my machine. That compiled and ran without issues for me.
I also tested the React Native OP-SQLite barebones app. This had some iOS lockfile issues, but after fixing those - the demo worked normally for me.