Summary:
Store immutable thrift-python isset bytes as a trailing tuple element instead of at index 0, so field values are always zero-based whether or not isset inspection is enabled. This removes the field-start offset from the Python and C++ runtime paths, updates the public inspection helpers to read the trailing bytes, and updates the Python CAPI codegen to emit zero-based tuple positions.

It also collapses the two immutable tuple container policies into a single `TupleContainer`, making the trailing isset array an orthogonal `WithIsset` template parameter on the container-creation helpers rather than a separate type.

Reviewed By: iahs

Differential Revision: D110348797

fbshipit-source-id: 4293c5998c54361599a29da992c5e421748fc7a5