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GH-38506: [Go][Parquet] Add NumRows and RowGroupNumRows to pqarrow.FileWriter (#38507)

### Rationale for this change

When using a chunked column reader to read from one Parquet file and a chunked column writer to write to another Parquet file, it can be useful to keep track of the number of rows written.

### What changes are included in this PR?

This branch adds a new `RowGroupNumRows` method to the `pqarrow.FileWriter`.  This is somewhat similar to the existing `RowGroupTotalBytesWritten` function.

### Are these changes tested?

A new `file_writer_test.go` file is added that adds a test for the new method.

### Are there any user-facing changes?

The new method is exported and documented.

* Closes: #38506

Authored-by: Tim Schaub <tim@planet.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>