Allow to set tags on BSR commits without changes

[trietsch]

Currently, when the latest version of a BSR repo is up to date with the changes being pushed to the BSR with buf push, no new tag can be created. This causes a misalignment between the tags in the BSR and our corresponding GitHub repository. Sometimes, changes to our GitHub repository must be done that do need a release, but do not contain any proto file changes. As we'd like to keep our GitHub repo in sync with the BSR in terms of tags, we cannot use the default buf push --tag <github_tag> all the time.

I'm aware of the CreateRepositoryTag rpc, though it might be nice if the CLI would facilitate this process.

[bufdev]

For future reference, this would likely be under a theoretical new buf beta registry tag add command.

[doriable]

We now have buf beta registry label create that allows you to create a label on the given commit. Closing this since this is now in 1.33.0.