[lib/keyserver/native] move all the reaction message types into its own file

Summary: I don't know why I didn't do this from the start, but moved ReactionMessageData RawReactionMessageInfo and ReactionMessageInfo into a reaction messages type file. Almost all the other message types do this as well

Test Plan: flow

Reviewers: atul, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6005