LUCENE-8635, adds support for loading FST offheap using mmap, users do not have the flexibility to specify fields for which FST needs to be offheap. This allows users to tune heap usage as per their workload.
Ideal way will be to add an attribute to FieldInfo, where we have put/getAttribute. Then FieldReader can inspect the FieldInfo and pass the appropriate On/OffHeapStore when creating its FST. It can support special keywords like ALL/NONE.