db: add sqlite/postgres as a database backend option

[Roasbeef]

Today we require etcd, which might be a bit more exotic to some devs. We should add support for sqlite+postgres via sqlc as we've done in some other projects.

We only need to store the onion private key, and also the macaroon root keys. With taproot assets, we have an implementation of the macaroon storage, so we can use that: https://github.com/lightninglabs/taproot-assets/blob/main/tapdb/macaroons.go