Clean up parquet_index example

[alamb]

Which issue does this PR close?

Follow on to https://github.com/apache/datafusion/pull/10549

Rationale for this change

When creating the example in https://github.com/apache/datafusion/pull/10549 I noticed a few APIs that were akward to use. Thus I am working on making PRs to improve them:

• [ ] FileScanConfig::new(): https://github.com/apache/datafusion/pull/10623
• [ ] ParquetExecBuilder

This PR shows how they will simplify the example of creating external indexes

What changes are included in this PR?

Update parquet_index.rs example to use simpler APIs

Are these changes tested?

Covered by CI

Are there any user-facing changes?