Add support for protobuf validation and add updated protos with constraints to the java client. Regenerate the code and apply any changes.

We have started to use validation constraints in our proto API specs. See https://github.com/project-kessel/relations-api/pull/102 

This requires the use of the `protoc-gen-validate` protobuf plugin. This may or may not be available locally. In the java case, a new dependency and some additional config is required in the pom.xml file to rebuild the base client.

Once this is plumbing is complete, the new protos must be added and any changes from code generation added to the client.

Acceptance criteria:

• Support added for protobuf validation to the pom.xml, such that validation annotations in the proto files are parsed without error.
  • Ideally those annotations would do client-side validation, but if it's not there in the generated code, we will not add it here.
• Updated proto files introduced in the above PR added to the client repo.
• Any changes arising from regenerating the files with `./mvnw clean compile` have been added to the client.