Switch to rustls and webpki-roots

This change replaces OpenSSL with rustls and also the manually curated
CA certs file with webpki-roots (effectively the same thing, but as a
crate).

Generally speaking the design of the network stack is the same. Changes:

- Code around certificate overrides needed to be refactored to work with
  rustls so the various thread-safe list of certificates is refactored
  into `CertificateErrorOverrideManager`
- hyper-rustls takes care of setting ALPN protocols for HTTP requests,
  so for WebSockets this is moved to the WebSocket code.
- The safe set of cypher suites is chosen, which seem to correspond to
  the "Modern" configuration from [1]. This can be adjusted later.
- Instead of passing a string of PEM CA certificates around, an enum is
  used that includes parsed Certificates (or the default which reads
  them from webpki-roots).
- Code for starting up an SSL server for testing is cleaned up a little,
  due to the fact that the certificates need to be overriden explicitly
  now. This is due to the fact that the `webpki` crate is more stringent
  with self-signed certificates than SSL (CA certificates cannot used as
  end-entity certificates). [2]

1. https://wiki.mozilla.org/Security/Server_Side_TLS
2. briansmith/webpki#114

Fixes web-platform-tests#7888.
Fixes web-platform-tests#13749.
Fixes web-platform-tests#26835.
Fixes web-platform-tests#29291.

tools/wptrunner/wptrunner/executors/executorservo.py

@@ -38,6 +38,7 @@ def build_servo_command(test, test_url_func, browser, binary, pause_after_test,
                         extra_args=None, debug_opts="replace-surrogates"):
     args = [
         "--hard-fail", "-u", "Servo/wptrunner",
+        "--ignore-certificate-errors",
         "-z", test_url_func(test),
     ]
     if debug_opts: