Add a way to test WebTransport with self-signed certificates

[vasilvv]

WebTransport API specification includes an option to connect to a host that uses a self-signed certificate, provided that the hash of that certificate is known in advance. The API requires the certificates that expire within two weeks, meaning that we can't just generate a certificate and check it into Git; we will have to dynamically generate a certificate when starting wptserve, and then provide the hash of that certificate to the scripts running.

cc @yutakahirano @bashi

[EternityForest]

I wonder if an exception could ve added to the protocol itself, allowing for arbitrary time limits so long as the connection is made via the local network or localhost.

[martenrichter]

Here is my attempt for the tests:
https://phabricator.services.mozilla.com/D231479
That I submit it via Firefox does not state a preference in browsers (I have contributed wt related fixes to Chromium and Firefox). They just managed to break it, which motivated me do something about it, since it breaks my infrastructure. So, if chromium contributed, I would like to join the review process....