Investigate automatic not null checks via annotations in place of checkNotNull calls

We've discussed that it would be neat if we could replace a lot of the checkNotNull() calls in parquet-mr with an annotation like
```Java
@NotNull
```

or even make not null the default and annotate things that can be null with
```Java
@Nullable
```

and have this enforced by the compiler / annotation preprocessor.

**Reporter**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain

_{**Note**: *This issue was originally created as [PARQUET-29](https://issues.apache.org/jira/browse/PARQUET-29). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by surveying checkNotNull() calls across parquet-mr and the project's current Java build configuration. Compare enforcing @NotNull by default versus annotating @Nullable values, including whether a compiler or annotation preprocessor is supported. Done means documenting a decided approach and its scope, or establishing why the change is not feasible.

Written by the indexing model from the issue text.