feat: add an optional config model for database retries

[sarumont]

Allows specifying base retry config values at the Database: level in a config file:

Database:
    DatabaseName: foobar
    Postgres:
        Address: localhost:5432
        ...
    Retries:
        MaxAttempts: 3
        MinDuration: "500ms"
        MaxDuration: "2s"

[adamdecaf]

Does this config need to be used?

[sarumont]

Yeah, I'm planning to use it for some PRs to the ledger services.