Use external trait codegen plugin (#93)

* Use external trait codegen plugin

* bump to 0.2

* remove deps

* bump

Author: kubukoz

project/PathRef.scala

@@ -16,4 +16,6 @@ addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.4")
 
 addSbtPlugin("com.disneystreaming.smithy4s" % "smithy4s-sbt-codegen" % "0.18.37")
 
+addSbtPlugin("org.polyvariant" % "smithy-trait-codegen-sbt" % "0.2.2")
+
 addDependencyTreePlugin