enhancement: caching of partition_schemas in `fn plan_files()`

[marvinlanhenke]

As of right now, the partition_schema is built in every iteration for each ManifestFile regardless if it has the same partition_spec.

This could be optimized as well by caching the partition_schema, similar to the partition filter.

Also the unnecessary clones (here, here) could be removed, or only be "lazily cloned" if we have a cache miss.