swarm/: Support generic connection management through NetworkBehaviour

[mxinden]

Description

This pull request enables generic connection management by implementing option (2) of https://github.com/libp2p/rust-libp2p/issues/2824:

Extend NetworkBehaviour with methods called to review a pending or established connection.

Very much work in progress at this point.

Links to any relevant issues

Open Questions

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