Support Parquet Generic Tables

### Is your feature request related to a problem? Please describe.

Currently, the Spark Plugin only supports Iceberg and Delta Lake tables. See the limitations [here](https://github.com/apache/polaris/tree/main/plugins/spark#limitations):
> For other non-Iceberg tables like csv, it is not supported today.

Based on the conversations [here](https://lists.apache.org/thread/652z1f1n2pgf3g2ow5y382wlrtnoqth0), we wanted to start off trying to leverage Generic Tables for the Parquet use case.

### Describe the solution you'd like

Have users be able to use Parquet files as generic tables.

### Describe alternatives you've considered

1. Have a Parquet Table Source. https://docs.google.com/document/d/1gR8UChF__KUz9h6fCCae1wv3b4PAdDTqZHJhdFBQ5iE/edit?tab=t.x0thpmlnrvwt#heading=h.ltz6ol3lh95k - This idea was not explored at this time.
2. https://docs.google.com/document/d/1ofljkrtiXRWc-v6hfkg_laKlYltepTPX7zsg44Tb-BY/edit?tab=t.0#heading=h.7ic5c343eju1 - Have table-like volumes. This has not been investigated at this time.

### Additional context

_No response_

Start with the Spark Plugin limitations linked in the issue, then review the Generic Tables context and the two alternative design documents. Done means users can use Parquet files as generic tables through the Spark Plugin, with the behavior and scope aligned with the selected design.

Written by the indexing model from the issue text.