Summary:
The patch adds a dedicated cache entry role for blob values and switches
to a registered deleter so that blobs show up as a separate bucket
(as opposed to "Misc") in the cache occupancy statistics, e.g.
```
Block cache entry stats(count,size,portion): DataBlock(133515,531.73 MB,13.6866%) BlobValue(1824855,3.10 GB,81.7071%) Misc(1,0.00 KB,0%)
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10601
Test Plan: Ran `make check` and tested the cache occupancy statistics using `db_bench`.
Reviewed By: riversand963
Differential Revision: D39107915
Pulled By: ltamasi
fbshipit-source-id: 8446c3b190a41a144030df73f318eeda4398c125