Support more message kinds as parameter types

[torkelrogstad]

Based on my reading of isWKTWithScalarJSONMapping and its usage, it seems like the only messages that's supported for parameters are primitive wrappers and google.protobuf universe. Would be great to also support more types from the google.type universe. I'm interested in using google.type.Date, at least.

[emcfarlane]

Hey @torkelrogstad you are correct, currently there is no custom formatting for JSON types (see this related issue that asks for google.type.Date). The parameter mapping uses the proto JSON representation and is restricted to these subset of types. Adding custom formats would have to be done with a custom JSONCodec. It would need to alter the methods MarshalAppendField and UnmarshalField to handle the date type marshalling. Hope that helps!