Introduce Velocity Template Engine to Parquet Generator

Much easier than the current setup of manually outputting the strings.

**Reporter**: [David Mollitor](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=belugabehr) / @belugabehr
**Assignee**: [David Mollitor](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=belugabehr) / @belugabehr

_{**Note**: *This issue was originally created as [PARQUET-1925](https://issues.apache.org/jira/browse/PARQUET-1925). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by locating the Parquet generator and the code that manually assembles its output strings; the issue does not name specific files or tests. Read the generator's current output path and determine the scope of introducing Velocity, then verify that generated output remains correct through the project's existing generator tests.

Written by the indexing model from the issue text.