Allow rotating usernames on NpgsqlDataSource in addition to passwords

We have mechanisms for rotating passwords, but some schemes require for rotating usernames as well. As an example, [see these Hashicorp docs](https://developer.hashicorp.com/vault/tutorials/db-credentials/database-secrets#define-a-username-template).

Raised by @beliakov-mb in https://github.com/npgsql/efcore.pg/issues/3193