Prototype implementation of javac plugin to serialize nullness annotations

[msridhar]

This is a new approach to serializing the annotations from https://github.com/jspecify/jdk for use with NullAway. The idea is we build that JDK with this plugin, which will dump out all the discovered annotation info into JSON files. Then, we write some code to take all those JSON files and convert them to astubx for NullAway. I went with JSON instead of directly serializing astubx to iterate more quickly. I have tested with jspecify/jdk and confirmed I can inject this plugin into the build and the output JSON files get created. I haven't started on the code to convert the JSON to astubx yet.

Of course there is much left to do, but I wanted to get this initial implementation up for review rather than dropping a huge PR at the end (and also to enable others to help out). We previously thought about getting out the JDK annotations by parsing the source files (and wrote some code to do so), but I think this approach will be more robust, as in the javac plugin we don't need worry about parsing of recent language features and all symbols are just resolved.

Related to #950 (which probably needs to be updated if we go forward with this approach)

[codecov]

Codecov Report

Attention: Patch coverage is 96.20253% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.44%. Comparing base (20dd152) to head (359bd3d).

Files with missing lines	Patch %	Lines
...away/javacplugin/NullnessAnnotationSerializer.java	96.20%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1225      +/-   ##
============================================
+ Coverage     88.36%   88.44%   +0.07%     
- Complexity     2366     2369       +3     
============================================
  Files            89       90       +1     
  Lines          7693     7772      +79     
  Branches       1540     1552      +12     
============================================
+ Hits           6798     6874      +76     
- Misses          450      452       +2     
- Partials        445      446       +1     

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[msridhar]

@cpovirk would be interested to get your thoughts on this approach

[cpovirk]

That all sounds reasonable to me. I don't want to obligate @cushon or @eamonnmcmanus to get deep into this, but if either of them wants to run screaming into the chapel to object, now would be the time :)