This user story is doable by the initial design (https://pulp.plan.io/issues/3405), but it currently requires users to make a number of preliminary calls to pulp to gather information that could be replaced with more arguments to a single call. My guess is that adding units by tag name and source repo will be a common use case. If this is necessary, we should write a separate story, but I'll write it out here for initial feedback. This story is to accept repository version hrefs and tag names. If they are provided, they should be used to generate a list of tag content units to add to a repository, and this list should be passed to the rest of the work implemented in 3405.