message: Forbid list/map as paylods
  * Forbid list/map as paylods
  ...and add tracking to the issue in unions in paylods
diff: @@ -272,6 +272,8 @@ class HttpBoundProtocolPayloadGenerator(
  """,
  )
  is StructureShape -> rust("#T()", serializerGenerator.unsetStructure(targetShape))
  + is UnionShape -> throw CodegenException("Currently unsupported. Tracking issue: https://github.com/awslabs/smithy-rs/issues/1896")
  + else -> throw CodegenException("`httpPayload` on member shapes targeting shapes of type ${targetShape.type} is unsupported")
  }
  }
  }
  