Support writing digest instead of tag with semver strategy

[otherguy]

Is your feature request related to a problem? Please describe. We're using the semver strategy for our production image updates, which works well. However, we also recently enabled Binary Authorization, which requires the use of image digests rather than tags.

Describe the solution you'd like I'm happy with the semver strategy as our production images are tagged in the semver format, but I would like ArgoCD Image Updated to be able to look up the digest for the image it's trying to update to, and instead write that back to git.