[native] introduce C++ message spec for reactions

Summary:
introduce C++ message spec for reactions. This missing C++ message spec was triggering an error pop up telling the user that there was an "Error setting threadStore or messageStore"

Linear Task: ENG-2645

Test Plan: Reproducing the original error is a bit hit or miss for me, but after these changes I haven't seen the error pop up since

Reviewers: atul, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6183