Migrate KNNEngine interfaces

[ajleong623]

Description

Migrates some of the required interfaces from the KNN plugin to core, avoiding repeated work between different KNN plugin implementations.

Related Issues

#20050

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for f1c9689: FAILURE

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❌ Gradle check result for db2fad8: FAILURE

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[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.