feat(swarm): introduce ConnectionId::new_unchecked constructor

[thomaseizinger]

Description

In earlier iterations of the design for generic connection management, we removed the ConnectionId::new constructor because it would have allowed users to create ConnectionIds that are already taken, thus breaking invariants that NetworkBehaviours rely on. Later, we incorporated the creation of ConnectionId in DialOpts which mitigates this risk altogether.

Thus, it is reasonably safe to introduce a public, non-deprecated constructor for ConnectionId that can be used for tests.

Related #3327 (comment).

Notes & open questions

cc @melekes

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

[melekes]

[thomaseizinger]

I am going to directly release this after it merges to unblock @melekes. This is trivial in my eyes, hence I am merging this right away.

[mergify]

This pull request has merge conflicts. Could you please resolve them @thomaseizinger? 🙏

[melekes]

@thomaseizinger @mxinden what is the timeline for the next patch release (0.51.2)? we're (Parity) interested in this change specifically. I can create a PR if necessary.

[mxinden]

I will try tomorrow. No promises @melekes.