prosodyctl: Add external connectivity check based on observe.jabber.network
This uses the (experimental) observe.jabber.network API to
perform external connectivity checks. The idea is to complement
the checks prosodyctl can already do with a (nearly) complete
s2s/c2s handshake from a remote party to test the entire stack.
Thanks for your interest in contributing to the project!
There are many ways to contribute, such as helping improve the
documentation, reporting bugs, spreading the word or testing the latest
development version.
You can find more information on how to contribute at <https://prosody.im/doc/contributing>
See also the HACKERS and README files.