[CommCoreModule] implement C++ DraftStore
Summary:
Grouping logic in one class to make it cleaner and reuse some code.
NOTE: I will wait with creating diffs for the rest of the stores until this is accepted because rest will be very similar to this one.
Depends on: D8603
Test Plan: Build iOS and Android and play with drafts to make sure functionality still works.
Reviewers: michal, jon, atul, tomek, marcin
Reviewed By: michal, atul, tomek
Subscribers: marcin, ashoat
Differential Revision: https://phab.comm.dev/D8604