Add keyring param to gpg modules, final cleanup

[lkubb]

What does this PR do?

• Adds keyring param to all functions (if sensible)
• Adds option to delete empty keyring file to gpg.absent
• Finally brings all functions up to speed by having user, gnupghome and keyring (if sensible)
• Adds functional tests for both modules
• Fixes some miscellaneous bugs surfaced by the tests
• Corrects documentation

Todo:

• add gnupghome and keyring to trust_key
• check why gpg.decrypt test is failing on older systems
• read the docs and correct some more errors

This is based on #63162, so that one would have to be merged before. PR has been merged.

What issues does this PR fix or reference?

Fixes: #59783

Previous Behavior

always uses default keyring

New Behavior

keyring can be specified

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

[lkubb]

@twangboy Can we still get this into 3007? This PR has celebrated its first birthday recently.