[Feature Request] Option to add OpenSearch Serverless collection as a Data source type

Hello team,

Initially I would like to be able to create an OpenSearch Collection, which is an OpenSearch Serverless resource via CDK. However according to this [documentation](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_appsync.OpenSearchDataSourceProps.html#properties), the API datasource properties are API, domain, description, name and serviceRole. The [domain property](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_appsync.OpenSearchDataSourceProps.html#domain) is the OpenSearch domain endpoint URL and not the OpenSearch Collection endpoint.

When I saw that this option was not available in CDK, I navigated to the AppSync Console and tried to create a test API (MyAppSyncApi), however I see the option to add an OpenSearch Serverless collection is not available under Data source types. Hoping to have the OpenSearch Serverless collection supported.

In the meantime, please let me know if there are any workarounds (if possible) to achieve the above use case

Start by reviewing the AWS CDK OpenSearchDataSourceProps documentation and the AppSync Console's data source types. Compare the existing domain-based OpenSearch support with the requested OpenSearch Serverless collection endpoint, and consider the workaround context in the issue. Done means AppSync can create and use an OpenSearch Serverless collection as a data source.

Written by the indexing model from the issue text.