parquet: Add support for user-provided metadata loaders #12592
Description
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