parquet: Add support for user-provided metadata loaders

### 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_

Trace ParquetFileReaderFactory, DefaultParquerFileReaderFactory, ParquetOpener, and ParquetExec, then follow the SessionState::register_file_format(..., true) registration path. Done means a custom factory can provide metadata-loading behavior, including caching, without reimplementing the opener and execution components.

Written by the indexing model from the issue text.