[UNDERTOW-2660] Add RoutingHandler usage example

[anamitraupadhyay]

This class demonstrates the use of RoutingHandler to handle GET and POST requests at the /greet endpoint, along with a fallback 404 handler.
Jira: https://issues.redhat.com/browse/UNDERTOW-2660

Summary: Adds a standalone example class (RoutingServer) under the examples module demonstrating RoutingHandler usage:-

GET /greet
POST /greet (reads request body)
Fallback 404 for unmatched paths
Details:

Example-only; no Undertow core changes
Includes current-year Apache 2.0 license header
Uses UndertowExample annotation for discovery via Runner

Open to relocating the example if maintainers prefer a different module/path
Tests: Not applicable for examples; existing RoutingHandler tests already validate handler behavior

Backwards compatibility: No API changes; no performance or security impact

[anamitraupadhyay]

7 commits are already squashed into 1 commit

[baranowb]

@anamitraupadhyay - could you please update commit messages to include [UNDERTOW-2660] as prefix ?

[baranowb]

Need proper prefix in commit message.
Also, those commits can be squashed into one?

SIDE NOTE: for some reason examples/src/main/java/io/undertow/examples/routing server/RoutingHandler.java but package is defined as io.undertow.eaxmples.routing - also file path/package has whitespace in it.

[anamitraupadhyay]

Need proper prefix in commit message.
Also, those commits can be squashed into one?

SIDE NOTE: for some reason examples/src/main/java/io/undertow/examples/routing server/RoutingHandler.java but package is defined as io.undertow.eaxmples.routing - also file path/package has whitespace in it.

All are fixed