[Feat]: Remove Jackson annotation and use protobuf as the serialization mechanism #487

The https://github.com/a2aproject/A2A project now uses the a2a.proto file as the main source of truth.

Since we are already generating classes from this proto file, and using those for grpc and rest, it makes sense to use this for jsonrpc too.

The generated classes are very verbose and thus not suitable for exposing to users, so we will keep the domain model classes in the spec/ module (although perhaps this should have a new name now) as the user API, and perform mapping between these classes and the generated ones.

Internally we will use our ProtoUtils class to convert between our domain model classes and the protobuf generated classes.

This means we no longer need to rely on Jackson for serializing the JSON files, so we can drop dependency on that, and remove the Jackson annotations from the domain model classes.

‼️ As part of this, we should consider documenting the use of ProtoUtils etc. to help other integrators be able to serialize.