Feature request: add support for git tags

[bergamin]

Same syntax as the PR tags, but to add a git tag on the source branch commit.

Reason: we use this task to automate the PR to master from our release branch. The build occurs based on the release branch and then it's merged into master. It would be very helpful if this task could also add an actual tag while doing that.

With this, current label could be renamed to "PR tags", while the new one could be called "Git Tags"

In case of duplicate tags, provide two behaviour options:

• Force the creation of the new one, deleting the older with same name
• Keep the old one and finish the task with a warning instead of an error