Add unix domain socket support

[agorgl]

Similar to PR #478, although this now uses the new Unix Domain Socket Support in Jetty through jetty-unixdomain-server
https://webtide.com/unixdomain-support-in-jetty/

Usage of the new APIs is documented here:
https://jetty.org/docs/jetty/11/programming-guide/server/http.html#connector
https://jetty.org/docs/jetty/11/programming-guide/client/http.html#transport-unix-domain

[agorgl]

Hmm, given that the unix domain sockets require at least java 16 (https://openjdk.org/jeps/380), should I update the pipeline to run the tests using java 17? I think (but i'm not sure) that this won't have any impact in runtime for the users that don't use the :unix-socket option

[weavejester]

Thanks for the patch. I'd like to be sure that Jetty works over TCP for older versions of the JVM. Can you wrap the failing test in a conditional that checks the JVM version? Then we can potentially check with both.

[agorgl]

Done! Please run the tests again

[weavejester]

It looks good. Can you ensure that the lines in this patch are 80 characters or under?

[agorgl]

Fixed a slightly longer def line in the test file, everything else is below 80 chars except from the unix-domain-server-connector function definition that I kept it to 91 to match the format of the server-connector function.
I saw that this slight exception occurs in other places too like the binding of queue-max-capacity in create-threadpool function further down, in favor of consistency.

[weavejester]

Aside from a small typo in the docstring, this looks ready to merge. Apologies for taking a while to review this.

[weavejester]

Thanks for the patch!