[lib] introduce SendReactionMessageRequest type to message types

Summary: introduce SendReactionMessageRequest type to message types. This type will be needed in the reactionMessageCreationResponder

Test Plan: flow and this will be further tested in subsequent diffs

Reviewers: atul, rohan, tomek, ashoat

Reviewed By: atul, tomek, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5752