Add Transient as a "connectedness" state

[Stebalien]

This will let users:

1. Quickly detect if the connection to a peer is transient.
2. Detect transitions from transient to "full" connections (and back) via events.

[Stebalien]

Needed for ipfs/boxo#526

[sukunrt]

Can we do?

Detect transitions from transient to "full" connections Can we do?
Detect transitions from transient to "full" connections (and back) via events.

I'm not sure if we can guarantee ordering between notifications for two different connections. The only guarantee right now is that disconnect on a connection is after connect. This behavior I believe was changed in https://github.com/libp2p/go-libp2p/pull/2373/files(and back) via events.

Ignore this. It does work correct for hole punching.