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)

• External Library Models: Adding support for Nullable upper bounds of Generic Type parameters #949
• Rename the Jar Infer Flags JarInferEnabled,JarInferUseReturnAnnotations  #940
• properly handle defaults for modeled @NullMarked classes (unannotated types are @NonNull)
• 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?