Allow getting a ParquetFileReader and the SchemaManifest from an Arrow.FileReader

[adamreeve]

• This allows getting a ParquetFileReader from an Arrow.FileReader, which might be required if you want to access things like the row group statistics
• In order to allow this in a safe way, I've added a new ChildParquetHandle class that holds a pointer to an object that is owned by some other disposable object, and added a new INativeHandle interface to support using either a ParquetHandle or ChildParquetHandle. There are other places in the code where we might want to use this, eg. the RowGroupMetaData returned from a RowGroupReader is one I noticed, but that should probably be a separate PR. See also Missing dispose of RowGroupMetaData in RowGroupReader? #322
• Additionally, to be able to interpret the Parquet metadata when the file has a complex nested structure, expose the SchemaManifest associated with a FileReader

[marcin-krystianc]

Nice 👍