cli: refactor logic for connecting to database

I'm quite sure I have seen this pattern in other CLI commands:
startBuiltinPostgres - codersdk.PostgresAuth - ConnectToPostgres

we may consider some base pattern for all commands using the database

_Originally posted by @mtojek in https://github.com/coder/coder/pull/17091#discussion_r2014730400_

We have a few commands now that can connect to the database directly and need to handle the conditional logic of builtin postgres:

- server
- server create-admin-user
- server regenerate-vapid-keypair

We should refactor and unify this.

We also have the `server dbcrypt` commands under `enterprise/` that could also benefit from this standardized logic.