fix(swarm): don't report NewExternalAddrCandidate if already confirmed

[stormshield-frb]

Description

Currently, NewExternalAddrCandidate events are emitted for every connections. However, we continue to get this event even when autonat has already confirmed that this address is external. So we should not continue to advertise the "candidate" event.

Notes & open questions

We have made the changes in the swarm instead of identify because it does not make it necessary to duplicate the ConfirmedExternalAddr vector in the identify Behaviour. Moreover, if any future Behaviour emit NewExternalAddrCandidate, the same rule will also be applied.

I had to edit the autonat_v2 tests which were always expecting a NewExternalAddrCandidate but the address was already confirmed.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Hi François, this LGTM! CC @umgefahren wdyt?

[drHuangMHT]

This PR is not marked send-it but ready to merge(branch out-of-date aside), any problem here? @jxs

[drHuangMHT]

cargo deny failed due to #5596; clippy(beta) failed due to too_long_first_doc_paragraph here:

rust-libp2p/misc/quick-protobuf-codec/src/lib.rs

Lines 14 to 16 in a2a2816

	/// [`Codec`] implements [`Encoder`] and [`Decoder`], uses [`unsigned_varint`]
	/// to prefix messages with their length and uses [`quick_protobuf`] and a provided
	/// `struct` implementing [`MessageRead`] and [`MessageWrite`] to do the encoding.