Introduce new annotation to skip fields for py-deprecated

Summary:
The thrift-py-deprecated codegen (especially when the json option is turned-on) cannot deal with maps/sets with structured keys.

Introduce a new (field-level) annotation to skip over problematic fields.

The next diff will modify the compiler to acutlaly skip over annotated fields.

Reviewed By: Filip-F

Differential Revision: D62617627

fbshipit-source-id: 2b79a1d0e99de1cc497a7cf8aed9574e55e2d085

thrift/annotation/python.thrift

@@ -30,6 +30,12 @@ namespace py thrift.annotation.python
 @scope.Definition
 struct Py3Hidden {}
 
+/// Hides in thrift-py-deprecated only
+@scope.Field
+struct PyDeprecatedHidden {
+  1: string reason;
+}
+
 @scope.Enum
 struct Flags {}