Feature Request: Signed tags

Now that we support signed commits (https://github.com/dolthub/dolt/issues/628), we should consider signed tags.

Signed tags have the advantage that you can have a seperate signing authority which can sign the data (a commit id) after the commit has been created. So you can do things like ingest a bunch of data and get a reproducible ID to test with before you bless the commit with a signed tag.

Git has this feature: https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-tags