[Backport 2.x] Integrates KNN plugin with ConcurrentSearchRequestDecider interface (…

[shatejas]

…#2111)

This allows knn queries to enable concurrency when index.search.concurrent_segment_search.mode or search.concurrent_segment_search.mode in auto mode. Without this the default behavior of auto mode is non-concurrent search

Signed-off-by: Tejas Shah shatejas@amazon.com (cherry picked from commit 0421cdc907b43e4a930bd5a51454e5efea8413b6)

Description

Backport for https://github.com/opensearch-project/k-NN/pull/2111

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 please fix the ITs.