Sync status refactoring (#5450)

As I was looking at the coupling between `SyncingEngine`,
`SyncingStrategy` and individual strategies I noticed a few things that
were unused, redundant or awkward.

The awkward change comes from
https://github.com/paritytech/substrate/pull/13700 where
`num_connected_peers` property was added to `SyncStatus` struct just so
it can be rendered in the informer. While convenient, the property
didn't really belong there and was annoyingly set to `0` in some
strategies and to `num_peers` in others. I have replaced that with a
property on `SyncingService` that already stored necessary information
internally.

Also `ExtendedPeerInfo` didn't have a working `Clone` implementation due
to lack of perfect derive in Rust and while I ended up not using it in
the refactoring, I included fixed implementation for it in this PR
anyway.

While these changes are not strictly necessary for
https://github.com/paritytech/polkadot-sdk/issues/5333, they do reduce
coupling of syncing engine with syncing strategy, which I thought is a
good thing.

Reviewing individual commits will be the easiest as usual.

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Co-authored-by: Dmitry Markin <dmitry@markin.tech>