SpringBoot Add public getUndertow method to UndertowWebServer

Change Summary

• getUndertow() to UndertowWebServer will be null until the server has been started. Issue #3016 would enable getUndertow() to behave in a similar way to getTomcat() on TomcatWebServer.

Issue

• https://github.com/spring-projects/spring-boot/issues/39839

Comment From: mhalbritter

Thank you!