[lib] Introduce MessageSpec.showInMessagePreview

Summary:
This diff introduces a generic way to support ENG-9556.

Depends on D13826

Test Plan:
Tested in combination with the rest of the stack:

1. Make sure membership operations (user joining / leaving) don't appear in MessagePreview
2. Make sure reactions to the viewer's messages still appear in MessagePreview
3. Make sure reactions to other user's messages don't appear in MessagePreview
4. Test fresh login to thread with only one message in the last 14 days, which is a reaction to a non-viewer message. Make sure the reaction initially appears in MessagePreview, but then is replaced after more messages are fetched by the client

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13827