Convex replicator module

[kobiebotha]

This adds support for Convex as a replication source. Since Convex itself is open source (technically also FSL), it was quite feasible for me to implement this.

As with any datastore, there are many quirks. I've attempted to document pertinent ones in a README.md in the module root. Required reading is the section titled "Mutation Transaction Atomicity".

To get a feel for the system, run the convex self-host-demo: https://github.com/powersync-ja/self-host-demo/tree/convex-demo. Then simply open two instances of the Convex React demo app (new) (http://localhost:3030) side by side.

When running the demo, to log into the Convex dashboard, you need to jump through some hoops:

1. Check the container logs for the convex-keygen Docker container
2. Get the "Admin key" printed to console

TODO

• Test against Convex Cloud (it has been a while since I did that)
• Fix replication metrics (currently reporting per transaction "page", should report per mutation?)
• Update Test Connection logic to ensure that the powersync_checkpoints table and write mutation function exist
• Measure replication performance
• Docs feat: Convex docs powersync-docs#456

[Rentacookie]

Looks good to me 😁

[rkistner]

I did not review all the details, but the approach looks good. The info in docs/convex is very useful! Should add similar docs for other dbs over time.