This commit configures Jetty for HTTP/2 support as soon as the following
conditions are met:

* `server.http2.enabled=true`
* Both `org.eclipse.jetty:jetty-alpn-conscrypt-server` and
`org.eclipse.jetty.http2:http2-server` are on classpath

This will use the Conscrypt library for ALPN and TLS support using
native libraries shipped within the Conscrypt uber Jar. This does not
require a JVM agent or patching the JDK classes.

Closes gh-10902