[Feature Request] Adding option to distable StandbyReplicas for OpenSearch Serverless

## Describe the solution you'd like

Reduce option to disable StandbyReplicas for OpenSearch Serverless on Knowledge Base.

## Why the solution needed

Standby replica is enabled by default, but in some cases e.g. PoC, users might want to reduce operational cost rather than availability.

## Additional context

doc: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-opensearchserverless-collection.html

## Implementation feasibility

Are you willing to discuss the solution with us, decide on the approach, and assist with the implementation?

- [x] Yes
- [ ] No

Start by reading the linked CloudFormation documentation and tracing where the Knowledge Base configuration is defined. Confirm how OpenSearch Serverless StandbyReplicas is currently enabled, then define the option's expected behavior and verify that a Knowledge Base can be created with standby replicas disabled.

Written by the indexing model from the issue text.