ParquetFileReader has a currentBlock information in a private field

The currentBlock variable is a metric information which can be useful to know which block the current record is being read from. If this variable has a getter, it would be possible to skip over a certain blocks.  

**Reporter**: [Tanuja Dubey](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=tanuja) / @tanuja5

_{**Note**: *This issue was originally created as [PARQUET-2125](https://issues.apache.org/jira/browse/PARQUET-2125). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start at the ParquetFileReader entry point and locate the private currentBlock field and the code that updates it while records are read. Check nearby reader tests and existing API conventions before exposing the block information. Done means callers can inspect the current block and use that information to skip blocks, with tests covering the getter and reading behavior.

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