Posthog-async

Changes:

1. Feature flag defaults have been removed
2. Setup logging only when debug mode is enabled.

3. Feature flag remote evaluation doesn't require a personal API key.

New Changes:

1. You can now evaluate feature flags locally (i.e. without sending a request to your PostHog servers) by setting a personal API key, and passing in groups and person properties to `is_feature_enabled` and `get_feature_flag` calls.
2. Introduces a `get_all_flags` method that returns all feature flags. This is useful for when you want to seed your frontend with some initial flags, given a user ID.

- Make poll_interval configurable
- Add `send_feature_flag_events` parameter to feature flag calls, which determine whether the `$feature_flag_called` event should be sent or not.
- Add `only_evaluate_locally` parameter to feature flag calls, which determines whether the feature flag should only be evaluated locally or not.

Breaking changes:

1. The minimum version requirement for PostHog servers is now 1.38. If you're using PostHog Cloud, you satisfy this requirement automatically.
2. Feature flag defaults apply only when there's an error fetching feature flag results. Earlier, if the default was set to `True`, even if a flag resolved to `False`, the default would override this.

- Support for sending feature flags with capture calls

- Support multi variate feature flags