Review GPG touch-points with `GitRepository`

[pjbgf]

Review the current state ahead of Bootstrap GA, including:

• Signing verification
  • Documentation
  • Implementation
  • Test coverage
• Signing commits
  • Documentation
  • Implementation
  • Test coverage

SSH Key signing is not yet supported. As part of the review above we should consider whether the current implementation for signing and verification are extensible enough to enable the support of SSH Key signing. Implementing SSH Key signing is not part of the scope within the milestone of this issue.

[makkes]

GitOps GA (as we have called it lately) has been reached so I'm moving this out of the "Bootstrap GA" milestone and into GA. We might need to revisit the GA milestone and what it should encompass.