[API] block-device 'add/remove_tag' operations use GET method, not POST

This should be fixed before we release 2.0.

FWIW, I think this may have been caused by poor naming in the @operation decorator: it takes a boolean argument "idempotent" which then exposes the operation via HTTP GET when idempotent is True or POST when False. However, mutations should always be done via HTTP POST, whether idempotent or not.

I'd debate whether the adjective 'idempotent' can even be applied to a GET. It suggests that an operation has happened but the end result is the same no matter how many times you do it. PUT is meant to be idempotent and in other REST APIs I've used, PUT is commonly used to update entities whereas POST is used to create them. MAAS' API widely violates that convention though so this is more a general comment than something we need to fix now.