AIP-136: custom methods should use parent or name

[toumorokoshi]

an example from AIP-136 reads:

rpc SortBooks(SortBooksRequest) returns (SortBooksResponse) {
  option (google.api.http) = {
    post: "/v1/{publisher=publishers/*}/books:sort"
    body: "*"
  };
}

instead of publisher, it should be one of the canonical name or parent, depending on the case. In this case it's parent, since it's referring to a collection-level operation.

Note we also need linter rules for ensuring only parent/name in the http path.