JSpecify subtyping checks for arrays

[msridhar]

Add checks for proper array subtyping (including nullability of array contents) at pseudo-assignments and for method overriding. We do not yet handle multi-dimensional arrays. Note that JSpecify allows for covariant array subtyping with respect to nullability (see https://github.com/jspecify/jspecify/issues/65) which complicates the implementation a bit. Also some hacks are required since javac does not propagate the array contents nullability annotation into the types of the relevant trees in all cases.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.45455% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 86.12%. Comparing base (f4c4734) to head (075fa41).

Files	Patch %	Lines
...ava/com/uber/nullaway/generics/GenericsChecks.java	96.22%	0 Missing and 2 partials :warning:
...away/src/main/java/com/uber/nullaway/NullAway.java	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #956 +/- ## ============================================ + Coverage 86.06% 86.12% +0.06% - Complexity 2027 2040 +13 ============================================ Files 81 81 Lines 6679 6708 +29 Branches 1287 1293 +6 ============================================ + Hits 5748 5777 +29 Misses 518 518 Partials 413 413 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.