[native] Introduce MessageEntity type alias in more places

Summary:
Noticed we had this in MessageStore.h and figured we could simplify some other places too.

Ended up moving it to DatabaseManagers so it could be used inside the libraries we package for web.

Test Plan: I tested compilation on both web and iOS

Reviewers: kamil, tomek, marcin

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12735