Custom methods

Rules for AIP-136, covering custom methods.

Rule name	Description
`http-body`	Custom methods must have the HTTP body set to `*`.
`http-method`	Custom methods must use the POST or GET HTTP verb.
`http-name-variable`	Custom methods should only use `name` if the RPC noun matches the resource.
`http-parent-variable`	Custom methods should only use `parent` if the RPC noun matches the resource.
`http-uri-suffix`	Custom methods should have a correct URI suffix.
`prepositions`	Custom methods must not include prepositions in their names.
`standard-methods-only`	Declarative-friendly resources should eschew custom methods.
`verb-noun`	Custom methods should be named with the verb, then the noun.

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