fix: accept interop peer records in identify and rendezvous

[Azzurra88]

Description

Accept both legacy and standard interop peer record envelopes in the identify and rendezvous decode paths.

This keeps signed peer records usable during the migration introduced by PeerRecord::*_interop without changing the default envelope encoder. It also adds focused tests for the core helper and both protocol consumers.

Notes & open questions

This only relaxes decoding on the consumer side. Changelog entries for libp2p-core, libp2p-identify, and libp2p-rendezvous may still be needed before opening a PR.

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, and thanks for this, but why would someone want to use both payload types?
#6204 did not introduce any migration, it just offered a function compatible with go and js libp2p payload types, but one can keep using the previous one if only interacting with rust-libp2p.
Can you share your usecase?