Handle @NullUnmarked methods with explicit @NonNull annotations in JDK javac plugin

[msridhar]

E.g.:

https://github.com/jspecify/jdk/blob/fd65be678b854e5d833ac3c8214d0051671c96a0/src/java.base/share/classes/java/nio/file/Files.java#L1821-L1823

We need to add library model / astubx support for null-unmarked methods, and generate the information about all the explicit @NonNull annotations.