litep2p/discovery: Publish authority records with external addresses only (#5176)

This PR reduces the occurrences for identified observed addresses.

Litep2p discovers its external addresses by inspecting the
`IdentifyInfo::ObservedAddress` field reported by other peers.
After we get 5 confirmations of the same external observed address (the
address the peer dialed to reach us), the address is reported through
the network layer.

The PR effectively changes this from 5 to 2.
This has a subtle implication on freshly started nodes for the
authority-discovery discussed below.

The PR also makes the authority discovery a bit more robust by not
publishing records if the node doesn't have addresses yet to report.
This aims to fix a scenario where:
- the litep2p node has started, it has some pending observed addresses
but less than 5
- the authorit-discovery publishes a record, but at this time the node
doesn't have any addresses discovered and the record is published
without addresses -> this means other nodes will not be able to reach us

Next Steps
- [ ] versi testing

Closes: https://github.com/paritytech/polkadot-sdk/issues/5147

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>