Introduce MimeTypes.Mutable.clear() API

[joakime]

Jetty version(s)
12.1.x

Enhancement Description
Introduce a simple MimeType.Mutable.clear() to help with establishing a fresh no-default MimeTypes for frameworks like spring-boot.

See: https://github.com/spring-projects/spring-boot/blob/bd6e3ea5cff16cc280b2986f42961eb4d3765372/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactory.java#L440-L446