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)

• #949
• #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?