chainHead: Track reported blocks to capture notification gaps (#5856)

There are cases during warp sync or re-orgs, where we receive a
notification with a block parent that was not reported in the past. This
PR extends the tracking state to catch those cases and report a `Stop`
event to the user.

This PR adds a new state to the RPC-v2 chainHead to track which blocks
have been reported.

In the past we relied on the pinning mechanism to provide us details if
a block is pinned or not.
However, the pinning state keeps the minimal information around for
pinning. Therefore, unpinning a block will cause the state to disappear.

Closes: https://github.com/paritytech/polkadot-sdk/issues/5761

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>