Add a way to only generate schema information for a given class

Today if a user annotates a class with annotations like ProtoTypeId it will generate a marshaller for them. A user may want to implement their own marshaller and if they do we still generate one. We may want a way to annotate to designate that the marshaller generation is not needed. We could also extend this to define in the annotation what the users' custom marshaller is and register it for them automatically in a generated context initializer so they don't have to like with the generated marshaller.

Start by tracing the ProtoTypeId annotation, generated marshaller path, and generated context initializer described in the issue. Determine how annotation metadata reaches generation and registration. Done means the issue's requested annotation behavior is defined, including suppressing generation and optionally registering a user-supplied marshaller automatically.

Written by the indexing model from the issue text.