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 https://github.com/ipfs/boxo/pull/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.