Http4sGrpcServicePrinter

Ported from org.http4s.grpc.generator.Http4sGrpcServicePrinter (http4s-grpc).

Carries the downstream prefixRoot fix: ScalaPB may already hand us _root_.-qualified type names (for example when flat_package or custom scalapb package options are in play). Blindly prepending _root_. then produced _root_._root_.foo.Bar, which does not compile. prefixRoot makes the qualification idempotent, and the package clause strips the prefix because package _root_.foo.bar is not valid Scala.