[lib] Introduce MinimallyEncodedThreadInfo type

Summary:
We need corresponding MinimallyEncodedThreadInfo for MinimallyEncodedRawThreadInfo. The only really relevant difference between the two is that MinimallyEncodedThreadInfo has $ReadOnlyArray<MinimallyEncodedRelativeMemberInfo> in members field instead of $ReadOnlyArray<MinimallyEncodedMemberInfo>.

Depends on D9845

Test Plan: Flow. Earlier in the stack I was writing unit tests for all of these, but I think the value of unit tests for these is questionable. Will defer for now and consider writing at the end.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D9846