Standard methods: Update

Rules for AIP-134, covering update methods.

Rule name	Description
`http-body`	Update methods must have the HTTP body set to the resource.
`http-method`	Update methods must use the PATCH HTTP verb.
`http-uri-name`	Update methods must map the resource's name field to the URI.
`method-signature`	Update RPCs should annotate an appropriate method signature.
`request-allow-missing-field`	Update RPCs on declarative-friendly resources should include allow_missing.
`request-mask-field`	Update RPCs must have a field mask in the request.
`request-mask-required`	Update RPCs must have a field mask in the request.
`request-message-name`	Update methods must have standardized request message names.
`request-required-fields`	Update RPCs must not have unexpected required fields in the request.
`request-resource-field`	Update RPCs must have a field for the resource in the request.
`request-resource-required`	Update RPCs must have a field for the resource in the request.
`request-unknown-fields`	Update RPCs should not have unexpected fields in the request.
`response-lro`	Declarative-friendly Update methods should use long-running operations.
`response-message-name`	Update methods must return the resource.
`synonyms`	Update methods must be named starting with "Update".
`update-mask-optional-behavior`	Standard Update `update_mask` field must be `OPTIONAL`.

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