Add a way to configure a different annotated JDK

[netdpb]

Right now the annotated JDK is included in the artifact, so the only way to use an alternative is to build locally. It would be good, especially for JSpecify, to make it possible to substitute an alternative annotated JDK.

[wmdietl]

This turned out to be already implemented:

checker-framework/framework/src/main/java/org/checkerframework/framework/stub/AnnotationFileElementTypes.java

Line 888 in feecad2

URL resourceURL = atypeFactory.getClass().getResource("/annotated-jdk");

looks up the annotated-jdk resources relative to the type factory.

jspecify/jspecify-reference-checker#188 uses this in the JSpecify reference checker.