Reintroduce Cross Context Dispatch in Jetty 12

[janbartel]

Implement cross context dispatch.

[joakime]

This PR is on track for 12.0.8 release, right?

[gregw]

@joakime @lachlan-roberts @olamy This is a big PR that we want in this release cycled (which may be rushed). Can you start reviewing this ASAP so it makes it in.

The key areas to consider are:

• How we find other contexts
• How we wrap other contexts as ServletContexts
• How we create a special RequestDispatcher from the wrapped cross context ServletContext
• The wrapping done before dispatch to turn eeX request/response back into core request/response
• The slight changes to wrapping done in ContextHandler and/or ServletContentHandler to handle cross context dispatch.
• Cross context params work because of the request attribute used by Fields.from, even if EE8/9 do their own parsing, they now use the Fields data structure
• Multipart params are a bit limited, specifically as the data structure is different, we can't do cross environment dispatch
• Cross context sessions work because of cross context cookies and and shared session id manager
• Cross context bypasses security handler

@joakime any thoughts on better testing all of this?

[joakime]

PR #11516 is needed for the testing additions I'm working on.

[olamy]

this is passing TCK. Need some Arquillian changes though. arquillian/arquillian-container-jetty#186

[joakime]

Merge this, it is quite good as it currently stands.
We'll keep adding testing it and refining over time.

[janbartel]

Should remove this System.err.println

[joakime]

Lets get this merged. It's a good change.