[keyserver][lib] Pass MessageData to generatesNotifs

Summary:
In a later diff, we'll use our new sidebarCreation parameter (introduced earlier in the stack) to affect whether notifs are generated or not.

Depends on D6796

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/D6797