HybridDirectory should open .dim files with mmap

[jpountz]

This is a follow-up to https://issues.apache.org/jira/browse/LUCENE-8932. The BKD index is now off-heap when the index input is a ByteBufferIndexInput. So if we want the index to move off-heap with the default directory implementation when upgrading to Lucene 8.4, we should make HybridDirectory open dim files with mmap.

@jdconrad I'm assigning it to you if you don't mind since you worked on the Lucene change

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Store)

[jdconrad]

@jpountz Happy to work on this. Will likely have follow up question soon :)

[jdconrad]

Closed by #49272