Support serialization of generated protobuf

[pvlugter]

Currently serialized state requires the reflective protobuf message, which includes the $type for creating the type url. This makes it awkward from TypeScript, where the reflective and statically generated types need to be mixed for state types.

Protobuf.js does now generate a getTypeUrl message (protobufjs/protobuf.js#1463).

This hasn't been released yet (no release for a year), although it looks work is being done to get a new version released. We could otherwise release from a fork temporarily, published under @kalix-io, to get this functionality.

[pvlugter]

New version of protobufjs has been published, so using only the statically generated protobuf types can be supported now.