feat: Add default Value type for EventBridgeEvent detail

[lmammino]

Issue #, if available: N/D

Description of changes:

When creating a new lambda using cargo lambda new and selecting the event type eventbridge::EventBridgeEvent you end up with broken code:

missing generics for struct aws_lambda_events::eventbridge::EventBridgeEvent expected 1 generic argument

This makes sense because an EventBridge detail event field is an arbitrary JSON object, so it could vary widely in shape and therefore it is implemented as a generic type that we need to provide ourselves.

So an easy fix would be something like:

// ...
use serde_json::Value;

async fn function_handler(event: LambdaEvent<EventBridgeEvent<Value>>) -> Result<(), Error> {
    // Extract some useful information from the request

    Ok(())
}

except serde_json also needs to be installed with cargo add serde_json, so there's a bit of friction for the user.

CloudWatch events are very similar (unsurprisingly since EventBrigde has spun off CloudWatch events, AFAIK) because they also have a generic detail field which can contain pretty much arbitrary content.

I noticed that the current implementation of the CloudWatchEvent provides a default type and that type is serde_json::Value.

I really like this approach because it provides a smoother starting point for the developer.

So, this PR proposes to adopt the same approach also to the EventBridgeEvent type and default detail to be of type serde_json::Value when no explicit type is provided for the associated generic type.

I hope this makes sense, but IMHO it can make things a little bit more frictionless when wanting to scaffold an eventbridge Lambda.

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[lmammino]

Hey @calavera, Is it worth doing a new release for this? Or do you have a specific cadence for that?