[epic]: payment db: SQL implementation & migration

[ziggie1984]

This is the tracking issue for the payments db SQL-ization project. It will be the go-to reference to see how the project is progressing.

I'll start with a check-list for easy reference & then will add any details & context lower down behind some drop-down tags.

• [ ] Prepare the code so it will be easy to use both backends and use the same test harness.

• [ ] Introduce SQL schema, queries & CRUD

• [ ] Implement the kvdb -> SQL migration for the relevant stores.

• [ ] Plug the new migration into the main code base & add itest jobs to run against them (behind a feature flag s.t they are not yet available in prod build).

• [ ] Once there is enough confidence in the code & migration, add the SQL impl & migrations to the main prod build