Add OpenFeature integration

[cmanallen]

Adds OpenFeature integration and extends the Scope with a flags property.

As flags are evaluated by an application they are stored within the Sentry SDK (lru cache). When an error occurs we fetch the flags stored in the SDK and serialize them on the event.

Some open questions:

• How do I seed the buffer's capacity from the SDKs (or Integrations) init arguments? Right now its hard-coded at 100. The buffer is stored on the scope so its hard to know which integration should be used.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
14257	1	14256	4336

View the top 1 failed tests by shortest run time

tests.test_utils test_get_current_thread_meta_failed_to_get_main_thread

Stack Traces | 0.061s run time

tests/test_utils.py:916: in test_get_current_thread_meta_failed_to_get_main_thread
    thread.join()
.../hostedtoolcache/Python/3.6.15........./x64/lib/python3.6/threading.py:1052: in join
    if self is current_thread():
.../hostedtoolcache/Python/3.6.15........./x64/lib/python3.6/unittest/mock.py:939: in __call__
    return _mock_self._mock_call(*args, **kwargs)
.../hostedtoolcache/Python/3.6.15........./x64/lib/python3.6/unittest/mock.py:1001: in _mock_call
    result = next(effect)
E   StopIteration

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[aliu39]

Why are we using 2 type hints here? First hint is for setup_once

[sentrivana]

Will have a proper look soon -- just two comments regarding tox for now