Make prefetchOnOpen configurable for index and/or data blocks

This came up in user discussions. It would be great to add an extra option to the CacheConfig that allows to specify what blocks are cached during region/file opening. This should allows to set BlockIndexOnly, BloomFilterOnly, AllIndexesOnly, DataOnly, and AllBlocks.

For large datasets it is not viable to load all blocks into memory, but to speed up access it still makes sense to prefetch the index blocks (being block index and Bloom filter blocks).