feat: extends IChannel with AsStream

[lucafabbri]

It implements a ChannelStream that wraps an IChannel.

The implementation does not touch the IChannel interface, even if some Stream abstraction can't be implemented without exposing some more information out of the IChannel interface.

Feature	Supported
Write	yes
WriteAsync	yes
Read	yes
ReadAsync	yes
Flush	no
Seek	no

[lucafabbri]

Refers #127

[flcl42]

Hello @lucafabbri ! The code looks great, just few notes:

Could you check src\libp2p\Libp2p.Core\Stream.cs? It contains some logic that may be useful for your implementation, it has more detailed handling of CanRead/CanWrite. it also implements such expected feature of streams: if there is noting yet to read it blocks until we have at least something. It returns immediately only if channel is closed for reading.
Could you remove that old implementation in favor of your one?