Jetty 12 : Simplify favicon.ico and jetty-dir.css behaviors

[joakime]

• Remove Server.getDefaultFavicon()
• Remove Server.getDefaultStylesheet()
• Remove MemoryResource
• favicon.ico is only managed by the DefaultHandler now
• DefaultHandler loads favicon as ByteBuffer now
• Introduce new ResourceService APIs.
  • void .loadStylesheet(Resource) - loads an arbitrary resource to be the contents of all jetty-dir.css requests
  • void .loadDefaultStylesheetIfNotPresent() - loads default stylesheet if not already present (done because ResourceService is not a LifeCycle)
  • boolean .isStylesheetRequest(String pathInContext) - to test if pathInContext is for a the stylesheet
  • boolean .sendStylesheet(String pathInContext, Request request, Response response, Callback callback) - to send stylesheet if appropriate
• Add Stylesheet testing in EE10 DefaultServletTest
• Add Stylesheet testing in EE9/EE8 DefaultServletTest
• Add Stylesheet testing in core ResourceHandler