Currently Pigeon unconditionally creates custom codecs for each API generated. If the API never uses custom data classes the custom codecs are subclasses of the StandardMessageCodec that add no new functionality. Since it is noisy and not useful we should elide those generated codecs. The *GetCodec functions could just return the StandardMessageCodec.

context and example extraneous codec generation: https://github.com/flutter/plugins/pull/4732/files#diff-3ebedc7e36a112756c08f3e113dfe67a770cfc86424713da23442fcf9b569b70R28-R47

Remember there is the flag --debug_generators if you want to have a quick way to link generated code to the generators code.

cc @tarrinneal