Add bash completion for services defined in ~/.pg_service.conf

[3ximus]

Description

I've added bash completion to services defined in the service file.

It's only triggered after typing service=

I've also cleaned some formatting of the file itself to make it consistent

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).

• I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)