CalVer as an additional update strategy

[r4vi]

This pull request introduces a new update strategy called calver for handling calendar versioning in the Argo CD Image Updater. The changes span across multiple files to integrate this new strategy, including documentation updates, code additions, and new tests.

Happy to take feedback - this is my first contribution here. The most controversial part of this PR is using allow-tags to provide a calver format as calver isn't really that standardised. It does feel slightly odd to me but the alternative was to introduce another annotation.