Tracking issue: making `jspecify/jdk` library models available to NullAway users

[msridhar]

This issue is for tracking remaining changes we need to make to use the jspecify/jdk library models with NullAway. We added some initial support for parsing source code and generating astubx files in #922, with support only for reading in @Nullable return types. (List below is quite incomplete for the moment)

• [X] #949
• [x] #1072
• [ ] #940
• [ ] properly handle defaults for modeled @NullMarked classes (unannotated types are @NonNull) #1006
• [ ] scripts to generate an astubx jar artifact that can be shipped to Maven Central
• [ ] add a flag for using the new models separate from JSpecify mode?

[msridhar]

We've changed our approach here rather significantly. We're now using a javac plugin to extract the nullness info out of jspecify/jdk. #1333 is our latest work on this approach, and it seems to be the way we will go.