low boilerplate solution for scalapb-validate integration

[jtjeferreira]

Short description

Provide an easy way to integrate scalapb-validate with an akka-grpc client or server

Details

Following https://github.com/akka/akka-grpc/pull/1248 and https://github.com/akka/akka-grpc/pull/1264 is there any plan to add validation to an akka-grpc client/server without much boilterplate (without using validate_at_construction)

Possible solutions:

• Do nothing. Users can still wrap the client or server and manually call the Validator
• Allow to "intercept" grpc calls and introduce an "interceptor" that does the validation like in the java client (https://github.com/envoyproxy/protoc-gen-validate/blob/main/java/pgv-java-grpc/src/main/java/io/envoyproxy/pgv/grpc/ValidatingClientInterceptor.java). Also see https://github.com/akka/akka-grpc/issues/711
• Change akka-grpc codegen to call the validation

[raboof]

Sounds like this could also be on top of something like #1433 - unfortunately I've been focusing on other work and haven't found time to seriously look into that one myself yet