update actions should have option to clear, remove, and add tags

[displague]

It should also be possible to add, remove, and clear tags.
This should be implemented in a common way across all resources with tags.

Would I expect this command to add a tag or reset all tags:

• metal thing update --tags foo // reset to one tag - foo?
• metal thing update --tags // clear all?
• metal thing update --tag foo // add a single tag foo? with no way to remove a single tag foo?
• metal thing update --tag -foo --remove-tag foo? Since there is no API call to remove a tag, would we full fetch and remove tags as part of update?

I think there is more to consider on tag setting. Happy to consider that outside of the scope of this PR and simply document the behavior this command uses for now.

metal thing update --add-tag=foo
metal thing update --add-tag-if-not-present=foo // do we go this far to avoid foo,foo,foo tags, which are possible iirc
metal thing update --remove-tag=foo

Originally posted by @displague in #369 (comment)