Add query group level rejection logic

[kaushalmahi12]

Description

With this change, we will be able to reject the requests for contended query groups.

Note

• The QueryGroupService is a stub here and will be replaced by the actual implementation in next couple of days
• Due to the above reason the tests for the QueryGroupService are intentionally left out to avoid the duplicate efforts.

Related Issues

RFC: https://github.com/opensearch-project/OpenSearch/issues/12342

Check List

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

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