Substream open/close functionality is not supported so they're left as
`todo!()s`.

Remove `SendNotification` command as sending notifications is now
supported by using the installed notification sink received in
`NotificationStreamOpened` event.

The role should not be part of `NotificationStreamOpened` as
`Notifications` has no knowledge of that and the protocols that are
interested in that information should query it from `Peerset` (when the
support for that is added)

Co-authored-by: Dmitry Markin <[email protected]>

Co-authored-by: Dmitry Markin <[email protected]>

The previous implementation of exposing `NotificationsSink` to the
protocol was too leaky. Introduce new `MessageSink` abstraction which
provides a clean send interface and internally handles notification sink
replaces so the protocol doesn't have replace the sink when it's replaced.

The original API ignores the case where the peer doesn't exist

Previously the peer role was decoded in `Protocol` but now that the
protocol is directly communicating with `Notifications`, the handshake
decoding is no longer done on behalf of the protocol.

Introduce a new function `NetworkPeers::peer_role()` which allows the
protocol to fetch the peer role. The function takes the received
handshake (which should contain the role) and in case the handshake
doesn't contain the protocol, it's fetched from `PeerStore`.

If the peer role cannot be determined, `None` is returned, indicating
to the protocol that it should discard the peer.

Co-authored-by: Dmitry Markin <[email protected]>