[proposal] Add Upgrade Support to JDKs HttpServer

[Christoph Läubrich]

I use the jdk http server quite often in my applications and came across 
a limitation recently that http upgrade is not supported.

As a proof of concept how an API/implementation might look like I create 
this PR:

- https://github.com/openjdk/jdk/pull/28961

it would be great to get some feedback if such support would be feasible 
and what are the next steps.

Looking forward for your comments,
Christoph