Make gpg resilient to host configuration changes

[holmanb]

Proposed Commit Message

fix(gpg): Make gpg resilient to host configuration changes

Use ephemeral GNUPGHOME in gpg commands.
Make a gpg context manager to manage daemon and gpg tempdir lifetimes.
Bring back process shutdown via gpgconf (fallback to killing when not present)
Add relevant tests and update existing tests.
Fixes several failing tests due to keyboxd changes in Noble.

Fixes GH-4989

Additional Context

failing tests fixes https://github.com/canonical/cloud-init/issues/4989

This is a lot bigger change than I expected, but most of the changes are in test updates and function argument and parameter changes.

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)