Add/Remove methods

Rules for AIP-144, covering add/remove methods.

Rule name Description
http-body Add/Remove methods should use `*` as the HTTP body.
http-method Add/Remove methods must use the POST HTTP verb.
request-message-name Add/Remove methods must have standardized request message names.

Note: Because AIPs sometimes cover topics that have some overlap, some rules related to add/remove methods may be included in the rules for other AIPs.