This commit mostly just moves some code and modules around for
better organization, but there are some functional changes.

- stream manager related code now lives in the 'stream' module.
- stream module exports public symboles using __all__ and __init__.py
  Users should import stream manager symbols like:
  `from eventutilities_python.stream import stream_manager, load_config`, etc.

- Stream.close has been removed from the interface.  Closing is handled
  by stream_manager.

- flink.py should contain only contain Flink specific things.  As such,
  EventStreamDescriptor has been removed (importing it would cause a circular import).
  This lower level APIs use stream_name and schema_version, similar to the Java interface.
  If we want to make EventStreamDescriptor a top level Java interface, we can.
  Or use and wrap java EventStream class instead.

Bug: T336488