Model Apache Flink's RichFunction.open as an @Initializer method

[lazaroclapp]

From the docs here: open(...) should always be called before other methods,
such as filter(...), so it fits our definition of an @Initializer method.

[lazaroclapp]

@msridhar let me know if you'd want a test case here. Would mean adding Apache Flink as a test dependency and there isn't really any new code path being tested, though (also, we are simultaneously making this patch as internal config at Uber while we wait for the next release here, will add you to the diff)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9092438) 87.02% compared to head (370cf08) 87.02%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
...ava/com/uber/nullaway/generics/GenericsChecks.java	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #862      +/-   ##
============================================
- Coverage     87.02%   87.02%   -0.01%     
  Complexity     1921     1921              
============================================
  Files            77       77              
  Lines          6219     6218       -1     
  Branches       1209     1209              
============================================
- Hits           5412     5411       -1     
  Misses          403      403              
  Partials        404      404              

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[msridhar]

I'm ok without a test for this one, though if possible it'd be good to do some kind of local sanity test (even just on the config is fine)