Add basic HTTP/1.1 support to the HTTP/2 Test Server

To more easily write HTTP Client test scenarios for different protocol
versions and different TLS versions, it would be very convenient if the
HTTP/2 server, that is part of the JDK regression test suite, was able
to issue basic HTTP/1.1 responses.

This enhance adds minimal support, which effectively returns a canned
200 - OK - response to all non-upgrade HTTP/1.1 requests, as well as
secure connections that do not negotiate the h2 ALPN. This new
functionality is opt-in so as to not inadvertently affect existing tests
which assume HTTP/2 only support.