Move the DefaultMetadataCache into its own module #19074
Description
Is your feature request related to a problem or challenge?
As the default implementations for cache's expand, the existing module structure for those implementations under datafusion/execution/src/cache/ have become a bit crowded.
Separating the cache implementations into separate modules was introduced here:
Describe the solution you'd like
The DefaultFilesMetadataCache implementation currently lives in cache_unit.rs
It could also be separated into its own module to help improve the organization of the cache module.
Describe alternatives you've considered
No response
Additional context
No response