Add in plugins for the motherduck and postgres extensions

[jwills]

Both of these extensions benefit from a bit of additional initialization logic that needs access to private (and thus profiles.yml-safe) config info.

1. For postgres, we likely want to do a call postgres_attach operation that uses a dsn string that main contain secrets (e.g. passwords),
2. For motherduck, we'll want to call pragma md_connect and possibly include the Motherduck token in the config (which again needs to be private.)