[DISCO-3405] feat: Support replicas for the redis adapter

[ncloudioj]

References

JIRA: DISCO-3405

Description

This allows Merino to connect to the replica endpoint of our Redis server to handle all the read workloads so that the primary node only needs to handle write workloads and replication related tasks.

This requires a new envvar for Merino for the replica endpoint. PR filed here.

PR Review Checklist

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• This PR conforms to the Contribution Guidelines
• The PR title starts with the JIRA issue reference, format example [DISCO-####], and has the same title (if applicable)
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• Documentation has been updated (if applicable)
• Functional and performance test coverage has been expanded and maintained (if applicable)

[ncloudioj]

Thanks for the review! The cloudops-infra PR has been merged. Let's give this a spin.