PathMappingsHandler exposes PathSpec and Context based on PathSpec

[joakime]

Jetty version(s)
12.0.7

Enhancement Description
The existing PathMappingsHandler does not record the matched PathSpec that lead to the decision to call the specific handler.

I propose we add the matched PathSpec to the Request attributes before calling the matched Handler.

This has 2 benefits.

1. The PathSpec details (like regex matched groupings, and uri-template variables / path-params, path-info, etc) can be obtained.
2. The Request.getPathInContext(Request) can participate in the smart resolution of path-in-context from the matched PathSpec (as a primary source if a ContextHandler is not present?)

[joakime]

Completed in PR #12738