[lib] introduce unshimMessageInfo to reaction message spec

Summary:
introduce unshimMessageInfo to reaction message spec. unshimMessageInfo handles the unshimming of unsuported messages to a supported message by simply returning the unwrapped messageinfo of the previously unsupported message"

Depends on D5690
Linear Task: ENG-2245

Test Plan:
Ran web and mobile app locally and nothing crashes/will be doing more tests on this message spec in subsequent diffs. Also in my local stack, (with some extra code that will come in subsequent diffs) I was able to get a reaction message to "unshim" which I feel is a strong indicator that this function works as expected (see screenshots below)

Reaction Message when unsupported:

Reaction Message when supported:

Reviewers: atul, rohan, tomek

Reviewed By: atul, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5805