[lib] Re-introduce ThreadInfo ([Legacy/MinimallyEncoded]ThreadInfo)
Summary:
Similar to eg D10077, but for ThreadInfo.
NOTE: web/native will be handled in separate diffs to make this a little easier.
Depends on D10095
Test Plan: CI/flow/etc
Reviewers: ashoat, tomek, ginsu, rohan
Reviewed By: ashoat
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D10122