Move metadata types to restate_types

[muhamadazmy]

Move metadata types to restate_types

Summary:
Moving all common metadata types that is used over grpc
to their own module under restate_types. This will make it
possible to reuse them for other grpc service (like NodeCtlSvc)

[muhamadazmy]

@tillrohrmann this work was done as a preparation for NodeCtlSvc work that should follow. This way we can add some metadata related functionality via the node svc which is required by the restatectl command to operate seamlessly. Please take a look when possible before new merge conflicts are introduced. Thank you