Renamed explanation response processor to hybrid_score_explanation

[martin-gaievski]

Description

Change the name of the new response processor from explanation_response_processor to hybrid_score_explanation. New name is more consistent with naming conventions for other search processors by dropping the response_processor suffix (https://opensearch.org/docs/latest/search-plugins/search-pipelines/search-processors/) and reflects the hybrid search specialization. Came up with the new name while working on the documentation PR for explainability opensearch-project/documentation-website#8645

with new processor name this is how the pipeline will look like:

{
    "description": "Post processor for hybrid search",
    "phase_results_processors": [
        {
            "normalization-processor": {
                "normalization": {
                    "technique": "min_max"
                },
                "combination": {
                    "technique": "arithmetic_mean",
                    "parameters": {
                        "weights": [
                            0.3,
                            0.7
                        ]
                    }
                }
            }
        }
    ],
    "response_processors": [
        {
            "hybrid_score_explanation": {}
        }
    ]
}

Related Issues

#905

Check List

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

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