[lib] Introduce MinimallyEncodedRawThreadInfo and validator

Summary:
RawThreadInfo type w/ minimally encoded fields.

Next diffs:

• Higher level utilities for translating back/forth from RawThreadInfo <=> MinimallyEncodedRawThreadInfo
• Native refactoring + migrations
• Web refactoring
• Flipping the switch

Depends on D9735

Test Plan: Single unit test, will be tested implicitly in subsequent diffs as well.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: tomek

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9736