feat(rust): add rendezvous service for nat hole punching

[neil2468]

Proposed changes

This is part of the prototype for UDP NAT Hole Punching (#3507).

• Add a 'Rendezvous service' to the UDP transport.
• Add tests to exercise new code.

Thanks!

Checks

• All commits in this Pull Request are signed and Verified by Github.
• All commits in this Pull Request follow the Ockam commit message convention.
• I accept the Ockam Community Code of Conduct.
• I have accepted the Ockam Contributor License Agreement by adding my Git/Github details in a row at the end of the CONTRIBUTORS.csv file in a separate pull request to the build-trust/ockam-contributors repository. The easiest way to do this is to edit the CONTRIBUTORS.csv file in the github web UI and create a PR, this will mark the commit as verified.

[mrinalwadhwa]

@neil2468 This looks awesome. Did you happen to test it across NATs somehow?

[neil2468]

@mrinalwadhwa Thanks. I've not yet tried setting up an evironment for NAT testing, but now that you mention it maybe I should do that next.

This PR does not implement hole punching yet, it's only adds the Rendezvous service which will maintain a table of the public IPs of remote nodes.

[davide-baldo]

Looks good!