Expose @EnsuresNonNull and @RequiresNonNull in annotations package

[msridhar]

We've supported limited versions of these annotations for a while but it's been undocumented. This PR puts a version of the annotations in our annotations module (so they will be part of the released annotations jar), improves the Javadoc, and also improves some of the corresponding error messages. We will also improve the wiki documentation for these annotations before the next release.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.95%. Comparing base (01651e4) to head (48bccf7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #999 +/- ## ========================================= Coverage 85.95% 85.95% Complexity 2073 2073 ========================================= Files 83 83 Lines 6872 6873 +1 Branches 1322 1322 ========================================= + Hits 5907 5908 +1 Misses 551 551 Partials 414 414 ```

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