[Backport 2.x] Preloads .vec and .vex files

[shatejas]

LuceneFlatVectorReader uses IOContext.Random to open the read. IOContext.Random indicates the kernel to not read ahead the pages on to physical memory. This causes an increase in merge time due to increase of read ops at runtime.

The preload settings signals the kernal to preload the files when the reader is opened

Description

Backport for https://github.com/opensearch-project/k-NN/pull/2186/files

Check List

• [x] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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[navneet1v]

@shatejas does additional settings didn’t work and we have to use additional index settings?