parquet: Add support for user-provided metadata loaders

[progval]

Is your feature request related to a problem or challenge?

This allows users to implement #12547 (caching metadata, especially the Page Index) themselves without reimplementing all of ParquetOpener and ParquetExec

Describe the solution you'd like

Add a method to the object returned by ParquetFileReaderFactory that loads metadata, so that users who overwrite the DefaultParquerFileReaderFactory with SessionState::register_file_format(..., true) can provide their own implementation.

Describe alternatives you've considered

Adding caching support directly in Datafusion. I tried in #12548 but it went nowhere.

Support for caching directly in Datafusion can be added later, based on this feature.

Additional context

No response