Scaffold MLS Server

[neekolas]

Summary

Uses the new protos from this PR and creates a new GRPC service.

The new service will only run if the --api.enable-mls and --mlsstore.db-connection-string options are set. Otherwise the server should continue to behave the exact same as normal.

The service is unimplemented, and the mlsstore package doesn't do anything, but it at least puts some of the pieces in place.

Notes

I am creating a new database for the service to hold the MLS group state and key package state to separate it from the regular message storage. There are a few reasons for this:

• Ensures that if we have unoptimized queries or bad migrations we won't break production while we are moving fast
• Makes it easier for us just to drop the database and recreate it if we have breaking schema changes
• Once the schema has stabilized we can always move the tables into the main DB if we think it makes sense
• It is a logically separate service with clear boundaries to the rest of the application. We are never going to need to join between this DB and the waku db