Support protoc-gen-validate

[bjaglin]

PGV makes it possible to define validation rules at the field level. In a RPC context, it's a great way to document the constraints and fail early if a rule is violated, without implementing anything.

In our gRPC services implementation, we chose to automatically return a INVALID_ARGUMENT gRPC error code on the server-side if anything in the request payload violates a rule, with a human-readable representation of the violation as a string.

In the context of a mock, PGV annotations could be enforced in a similar way to mimic this behavior:

1. at stubbing time, by returning a 400 HTTP response if anything in the request or response payload violates a rule
2. at serving time, by returning a INVALID_ARGUMENT gRPC error code if anything in the request payload violates a rule

By making (2) configurable (error code, message?), clients of gripmock can then be sure that validation rules will match how the actual implementation will behave.