Http4sGrpcServicePrinter

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

Every type name comes from ctx.fullName, which is the same call mill-protoc's message printers make, so the two targets cannot disagree about where a message landed. ctx.fullName already returns a _root_.-qualified name; 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.