client/finality-grandpa/src/observer: Poll NetworkBridge (#4766)

Patch d9837d7d reintegrated the periodic neighbor packet worker, by
moving its logic into a `Future` implementation on `NetworkBridge` which
needs to be polled by its upper layer.

Polling by the upper layer was implemented within the `Future`
implementation of the `VoterWork` struct but not within the `Future`
implementation of the `ObserverWork` struct. This patch adds polling of
the `NetworkBridge` to the latter.