Where should we stream FST to disk directly?

### Description

Most of the use cases with FST seems to be writing the FST eventually to a DataOutput (is it IndexOutput?). In that case we are currently writing the FST to an on heap DataOutput (previously BytesStore and now ReadWriteDataOutput) and then save it to the on disk.

With #12624 it's possible to write the FST to an on disk DataOutput. So maybe let first compile a list of places which can be migrated to the new way?

Note: With the new way, there is a catch: We can't write the metadata in the same DataOutput as the main FST body, it has to be written separately.