[lib] Introduce MinimallyEncodedThreadCurrentUserInfo and validator

Summary:
ThreadCurrentUserInfo type w/ permissions as string instead of ThreadPermissions.

Next diffs:

• Introduce MinimallyEncodedRawThreadInfo
• Higher level utilities for translating back/forth from RawThreadInfo <=> MinimallyEncodedRawThreadInfo. Found this to be cleaner API than encoding/decoding RawThreadInfo.currentUser.permissions and RawThreadInfo.members[memberID].permissions "manually" a bunch of different places.
• Native refactoring + migrations
• Web refactoring
• Flipping the switch

Depends on D9731

Test Plan: Unit tests, will be tested implicitly in subsequent diffs as well.

Reviewers: ashoat, rohan, ginsu, tomek

Reviewed By: ashoat

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9734