[native] Create feature flags context

Summary:
This context will be responsible for fetching and providing a configuration. At this point it just returns the default one.
Initially I tried to introduce this context in lib, so that it can be used by web, but that's not trivial, as this context requires staff flag that we currently compute on native and moving it to web also requires some thought as we e.g. hardcode isStaffRelease flag which doesn't make sense on web. Also, the initial release of feature flags service covers mobile only.

Depends on D6947

Test Plan: Consume the context in the app and console log it.

Reviewers: kamil, bartek, inka

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6948