SpringBoot Remove hardcoded knowledge of web server customisation when creating management context

https://github.com/spring-projects/spring-boot/blob/3b8148e10eea12335f627459b9dea6e3cba68096/spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/servlet/ServletManagementChildContextConfiguration.java#L127-L132

https://github.com/spring-projects/spring-boot/blob/3b8148e10eea12335f627459b9dea6e3cba68096/spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/reactive/ReactiveManagementChildContextConfiguration.java#L79-L85

Rather than this hardcoded knowledge, we want the customizers to be contributed via a plug point.

This is somewhat related to:

• https://github.com/spring-projects/spring-boot/issues/14302
• https://github.com/spring-projects/spring-boot/issues/9560
• https://github.com/spring-projects/spring-boot/issues/34926
• and others (see links from #9560)