fix(httpx): Prevent Sentry baggage duplication

[szokeasaurusrex]

Sentry baggage will get added to an HTTPX request multiple times if the same request is repeated. To prevent this from occurring, we can strip any existing Sentry baggage before adding Sentry baggage to the request.

Fixes #3709

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
13722	1	13721	4118

View the top 1 failed tests by shortest run time

tests.integrations.asgi.test_asgi test_auto_session_tracking_with_aggregates

Stack Traces | 0.094s run time

.../integrations/asgi/test_asgi.py:413: in test_auto_session_tracking_with_aggregates
    assert session_aggregates[0]["exited"] == 2
E   assert 1 == 2

To view more test analytics, go to the Test Analytics Dashboard
Got feedback? Let us know on Github

[sentrivana]

Found a typo. Overall looks good to me. Can we also add an httpx test specifically for the scenario where we were accumulating baggage?