[lib] Introduce MinimallyEncodedRoleInfo and validator

Summary:
RoleInfo type with permissions as $ReadOnlyArray<string> instead of ThreadRolePermissionsBlob.

Next diffs:

• Introduce MinimallyEncodedThreadCurrentUserInfo
• 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

Test Plan: Added some unit tests, will consume types in subsequent diffs.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9731