[lib] Add sidebarCreation flag to text and media MessageDatas

Summary:
This flag won't be passed on through to the RawMessageInfo. Instead, it's used by keyserver code that calls createMessages to be able to affect notification behavior deeper in the callstack.

More details in ENG-3041.

Depends on D6795

Test Plan:
I tested this whole stack as follows:

1. Create a sidebar by sending a text message on web
2. Create a sidebar by sending a media message on web
3. Create a sidebar by sending a text message on native
4. Create a sidebar by sending a media message on native

In all of these cases, I confirmed that only one notif was delivered (the notif for the text/media message was skipped)

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6796