SpringBoot Add customizer callback for WebHttpHandlerBuilder

Add a new interface for customizing the WebHttpHandlerBuilder before the HttpHandler is built from it.

For the test, there is also the alternative to use a customizer adding a filter and then take AssertJ and some InstanceOf checks to see, if the filter was added to the chain. It works as well, but seemed a bit long in code to me. If you think, this is a better test, hit me up and I will change it.

See gh-39324

Comment From: scottfrederick

Thanks for another great contribution @BenchmarkingBuffalo!