[lib] Introduce stripMemberPermissionsFromRawThreadInfo

Summary:
This utility will be helpful in rawThreadInfoFromServerThreadInfo and client migrations.

Depends on D12758

Test Plan: Added basic unit test, will continue testing implicitly with rest of stack.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12763