[native] Add a C++ MessageSpec for SidebarSource

Summary:
In D6869, I updated SidebarSourceMessageSpec.generatesNotifs so that it returned pushTypes.NOTIF. While SidebarSourceMessageSpec.notificationTexts should never be called, this still made it so SidebarSource messages could be included as the second item in notif payloads' messageInfos, since they get collapsed into the CreateSidebar.

As a result of being included in the notif payloads, SidebarSource is now getting processed by NSE, but since there is a missing C++ MessageSpec for it, we end up persisting an empty content to SQLite.

Test Plan: I was able to repro the "content must be defined" error before this diff by creating a sidebar that auto-added a test user. The test user would get the payload with SidebarSource, which would trigger the error. After this diff, the error no longer repros

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7085