Introduce control_flow_util_v2.py

This new module includes:
* FuncGraph subclasses for different kinds of control flow. This can
  be used to determine if the current graph is building control flow.

* in_defun() method to determine if the containing context should
  follow defun or v1 graph semantics.

* Shared functionality between cond_v2 and while_v2.

This functionality will be used in future changes. I made a new file
rather than reusing control_flow_util.py due to import cycles. If/when
we get rid of the old control flow, these files can be consolidated.

PiperOrigin-RevId: 217235279