This commit provides the scaffolding for using the new sql stores.
The new interfaces, structs and methods are in sync with other projects
like Taproot Assets.
- Transactional Queries: the sqldb package defines the interfaces required
to execute transactional queries to our storage interface.
- Migration Files Embedded: the migration files are embedded into the binary.
- Database Migrations: I kept the use of 'golang-migrate' to ensure our
codebase remains in sync with the other projects, but can be changed.
- Build Flags for Conditional DB Target: flexibility to specify our database
target at compile-time based on the build flags in the same way we do
with our kv stores.
- Update modules: ran `go mod tidy`.
