feat: add raw connection and transport tag to Connection interface

[HexaField]

Adds the ability to optionally expose the raw connection of a transport as well as to provide a transport identifier tag in the Connection interface.

Changes:

MultiaddrConnection type now has optional transportTag parameter
Connection class now has optional rawConn and transportTag parameters

Solves in part libp2p/js-libp2p#838

[vasco-santos]

@HexaField sorry for the late review. Let's get to add this feature now.

Per libp2p/js-libp2p#838 (comment) we should expose the multiaddrConn which will include the raw connection underneath.

[jacobheun]

For the connection we should just expose the multiaddr connection instead of its underlying raw connection. I don't think we need the transportTag, we should be able to inspect this via the multiaddresses.

[lidel]

Closing this PR as we'd like to just expose the underlying connection per libp2p/js-libp2p#838 (comment)