PostgreSQL connection: Username as fallback for database

[V02460]

This PR works on the PgConnectOptions to implement the fallback to the username if no database name is given. It notably touches the passfile lookup and connection establishment.

The first commit is a bit broader refactor to use getter methods for fields where appropriate. This better distinguishes raw user-supplied values vs. their derived form.

Is this a breaking change?

Yes. The following changes in observable behavior are intended:

• Passfile lookup defaulting to the username when not given a database name (breaking, did default to the empty string)
• Always passing a database name when establishing a connection (breaking, did pass None on missing database)
• Changed signature of get_database (breaking)
• New public getter methods get_password and get_extra_float_digits (not breaking)

[abonander]

The fallback to the username is actually done server-side. I don't think this change needs to be made, and it could be surprising to existing users of the API.

If anything, maybe we just need a docs clarification here.