Make the structure of the Undertow support and the structure of the Tomcat and Jetty support consistent

[wilkinsona]

Both JettyEmbeddedServletContainerFactory and TomcatEmbeddedServletContainerFactory have very few fields and the customisation required to apply the configuration in ServerProperties.Jetty and ServerProperties.Tomcat is applied using customisers. In comparison, UndertowEmbeddedServletContainerFactory contains numerous fields that duplicate those in ServerProperties.Undertow and it handles its own customisation.

I think this inconsistency should be removed, and I think that should happen by changing the Undertow support to align with the Jetty and Tomcat support. At the same time, we could consider moving the customisation logic out of ServerProperties.Jetty and ServerProperties.Tomcat into separate classes still in the spring-boot-autoconfigure module.

[snicoll]

Paging @bclozel as I think we might have fixed this one in the meantime.