arrow

GH-34086: [C++][Parquet] Fix writing num_rows to data page v2 (#34096)

### Rationale for this change

The C++ parquet writer does not correctly fill num_rows field to DataPageV2 header.

### What changes are included in this PR?

ColumnWriter keeps track of number of rows buffered in the current data page and then fills it into header of data page v2.

### Are these changes tested?

A test case has been added to make sure the data page header has been set correctly for required, optional and repeated columns.

### Are there any user-facing changes?

No.
* Closes: #34086

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>