Add engine-specific syncing

[MatthewTighe]

Synced tabs are being introduced to the home screen in Fenix. This exposes the need to be able to send sync requests for a specific SyncEngine on an as-needed basis, to ensure that the latest data is always available. This is because it doesn't always make sense to initiate an sync for the entire sync dataset - syncing history can be much slower than syncing tabs, for example.

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