[7.1]Docs: add example how to use custom ServletExtension implementation in Undertow

We should create some section that describes how to implement custom ServletExtension for Undertow and some example how to use it in EAP. As a reference you can use upstream doc.

Link to that section should be added to the org.apache.coyote.Constants.USE_CUSTOM_STATUS_MSG_IN_HEADER property mapping in Table A.1. Map Servlet Container and Connectors System Properties in EAP Migration Guide.