v1.2.1 - Fix PolicyRule sources/destinations type

Bug Fixes

• PolicyRule sources / destinations — Fixed incorrect type that caused 400 "couldn't parse JSON request" when creating/updating policies.

  The NetBird API expects plain string group IDs on writes but returns full objects on reads. Changed from List[Dict] to List[Union[str, Dict]].

  Before (broken):

  rule = PolicyRule(
      sources=[{"id": "group-id"}],  # 400 error
      ...
  )

  After (correct):

  rule = PolicyRule(
      sources=["group-id"],  # works
      ...
  )