Introduce MMKV to iOS app and NSE

Summary:
This differential introduces MMKV to iOS app. At this point there are only methods to set and get string, but future diffs will add more methods. This
differential introduces simplest API we can use and test.

Test Plan:

1. Below the comm::CommMMKV::initialize() line add line that sets value under a string key.
2. In NotificationService.mm at the beginning add line that tries to get string from the same key and logs this key in case it is present.
3. Send notification and check it the same string is logged.

Reviewers: tomek, bartek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11045