Map interface{} RPC projections correctly

A property whose type is `interface{}` in the IDL ought to be projected
as a "JSON-like" map, just like it is on the Coconut package side of things,
which means a `map[string]interface{}`.

pkg/tools/cidlc/gen_rpc.go

@@ -454,7 +454,8 @@ func (g *RPCGenerator) GenTypeName(t types.Type) string {
 			contract.Failf("Unrecognized GenTypeName basic type: %v", k)
 		}
 	case *types.Interface:
-		return "resource.PropertyMap"
+		// interface{} properties become "JSON-like" maps.
+		return "map[string]interface{}"
 	case *types.Named:
 		obj := u.Obj()
 		// For resource types, simply emit an ID, since that is what will have been serialized.