Http4sGrpcServicePrinter

Ported from org.http4s.grpc.generator.Http4sGrpcServicePrinter (http4s-grpc), with ScalaPB's DescriptorImplicits replaced by mill-protoc's GenContext.

Message type names come from messageType, which resolves them the same way mill-protoc's own printers do - ctx.fullName for a generated message, WellKnownTypeMappings for one of protobuf's - so the two targets cannot disagree about where a message landed or what it was mapped to. Those names are already _root_.-qualified; prefixRoot exists for the names this printer builds itself out of a package and a simple name, and is idempotent so that qualifying twice is not a way to emit _root_._root_.foo.Bar - the upstream bug this port was forked to fix.