[tag-mutation-project] consolidate tag mutation client API

[jackie-ob]

The initial release of tag mutation will include the following methods:

• A - Run.(add|remove|replace)_tag (singular)
• B - Run.(add|remove|replace)_tags (plural).

For internal backwards compatibility reasons, A methods accept iterables of tags also.

At some point we will migrate all internal users off of plural usage of A methods, and remove support for iterables of tags in A (use B instead).