AppSync EventAPI - Add DataSource - Lambda, EventBridge buses

### Name of the resource

AWS::AppSync::DataSource

### Resource name

AWS::AppSync::Api

### Description

## Describe the feature

Same fashion that we can add DataSources like Lambda functions, EventBridge buses, and more, to AppSync GraphqlAPIs; think it would be really good if we could also attach data sources to AppSync EventAPIs.

And also attach an EventBridge bus as a publisher to a AppSync EventAPI, therefore we could send events from multiple services to clients via a bus.

## Use Case

Feels like current AppSync EventAPI is missing a way to systems behind the client apps to react to subscriptions, connections and message publishes because they lack the capability to have datasources like a Lambda function or EventBridge bus.

Right now, we can:
1. Allow connected and subscribed clients to talk among them by publishing messages to namespaces and channels.
2. Fan out messages to connected and subscribed clients through EventBridge with ApiDestinations and Connections

But, it would be awesome if for example, we could add AWS Bedrock in the middle, triggering some AI work, and then spawning all subscribed clients with the result once it's ready.

## Extra Request

Would be nice if incoming events apart of bringing the event detail itself, could say which connection id sent it, and to which namespace/channel. That would empower so many other use cases.

## References

https://github.com/aws/aws-cdk/issues/33458#issuecomment-2661147045

Thank you,
LuisK

Start by reviewing the AWS::AppSync::DataSource and AWS::AppSync::Api resource names in the issue, then follow the linked AWS CDK discussion for context. Clarify whether the scope includes Lambda data sources, EventBridge bus publishing, and connection or namespace metadata. Done should mean that the supported AppSync EventAPI data-source capabilities and acceptance criteria are explicitly defined.

Written by the indexing model from the issue text.