Add BILLING_EVENTS_RESPONSE feature

[Hazel-Datastax]

What this PR does:
Add BILLING_EVENTS_RESPONSE feature

Response sample header:

Billing-Events | [{"id":"45edc7aa-b4ad-4f60-8184-10fba15f66a4","timestamp":"2026-06-02T22:43:27.836646Z","product":"serverless","event_type":"internal_model_total_tokens","properties":{"usage":7,"region":"CASSANDRA_REGION","resource_type":"serverless_database","resource_id":"SINGLE-TENANT","provider":"nvidia","model":"nvidia/nv-embedqa-e5-v5"}},{"id":"bb1ef231-b728-4253-a4b4-d3c9d161fffa","timestamp":"2026-06-02T22:43:27.836659Z","product":"serverless","event_type":"internal_model_egress_bytes","properties":{"usage":84,"region":"CASSANDRA_REGION","resource_type":"serverless_database","resource_id":"SINGLE-TENANT","provider":"nvidia","model":"nvidia/nv-embedqa-e5-v5"}},{"id":"787289e9-a936-4f70-ac70-5d7354b57f1b","timestamp":"2026-06-02T22:43:27.836661Z","product":"serverless","event_type":"internal_model_ingress_bytes","properties":{"usage":21832,"region":"CASSANDRA_REGION","resource_type":"serverless_database","resource_id":"SINGLE-TENANT","provider":"nvidia","model":"nvidia/nv-embedqa-e5-v5"}}]
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Which issue(s) this PR fixes:
Fixes #

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CLA Signed: DataStax CLA