HTTP and gRPC Transcoding

Rules for AIP-127, covering http and grpc transcoding.

Rule name	Description
`http-annotation`	HTTP annotations must be present on non-streaming methods.
`http-template-pattern`	HTTP template variable patterns should match the patterns defined by their resources.
`http-template-syntax`	HTTP patterns should follow the HTTP path template syntax.
`resource-name-extraction`	HTTP annotations should extract full resource names into variables.
`uri-leading-slash`	URIs should always begin with a leading slash.

Note: Because AIPs sometimes cover topics that have some overlap, some rules related to http and grpc transcoding may be included in the rules for other AIPs.