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GH-41159: [Go][Parquet] Improvement Parquet BitWriter WriteVlqInt Performance  (#41160)


[GH-41159](https://github.com/apache/arrow/issues/41159)
### Rationale for this change

This change improves Parquet FileWriter performance while writing parquets from arrow Records.
We saw a speed improvement from writing 320k rows/sec -> 650 rows/sec after making this change.

### What changes are included in this PR?
This PR reuses the `buf` variable being used by the bitWriter when writing parquet files.

### Are these changes tested?
Yes

### Are there any user-facing changes?
No

Authored-by: @ hhoughgg 

* GitHub Issue: #41159

Lead-authored-by: Andy Fan <duan-wei@cloudflare.com>
Co-authored-by: andyfan <52736754+DuanWeiFan@users.noreply.github.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>