Firestore emulator doesn't emit all CloudEvent properties

Hello there, thank you for making this!

I'm trying to test my Firestore triggered function (as shown in the `protobuf/rawevent` example) against the Firestore emulator.

I've managed to wire the emulator up the point where I can receive events to my function (by using [this](https://github.com/GoogleCloudPlatform/functions-framework-python/issues/104#issuecomment-758954342) comment).

It seems that the emulator doesn't respect the CloudEvent interface.

The error I get on receiving the event to my function (`oncreatetodo` is the name of the function, I've changed it from default `function`):

```
Listening on :5001
2024-09-29T15:05:53.355871 0:31:05.307182 POST [400] /functions/projects//triggers/oncreatetodo
Could not decode the request as a structured-mode message. (400)
CheckedFromJsonException
Could not create `CloudEvent`.
There is a problem with "id".
Required keys are missing: id, source, specversion, type. (CheckedFromJsonException)
package:json_annotation/src/checked_helpers.dart 66:5 $checkedNew
package:json_annotation/src/checked_helpers.dart 30:10 $checkedCreate
package:functions_framework/src/cloud_event.g.dart 11:63 _$CloudEventFromJson
package:functions_framework/src/cloud_event.dart 57:7 new CloudEvent.fromJson
package:functions_framework/src/targets/cloud_event_targets.dart 105:23 _decodeValidCloudEvent
package:functions_framework/src/targets/cloud_event_targets.dart 80:10 _decodeStructured
===== asynchronous gap ===========================
package:functions_framework/src/targets/cloud_event_targets.dart 53:19 CloudEventWithContextFunctionTarget.handler
===== asynchronous gap ===========================
package:google_cloud/src/logging.dart 56:26 _handleBadRequest.
===== asynchronous gap ===========================
package:google_cloud/src/logging.dart 64:24 _errorWriter.
```

What would be the ideal solution to be able to test locally before I deploy?

`EmulatedCloudEvent` (all optional properties) that is used with a testing flag when running the function?
Firestore emulator respects CloudEvent interface so it's possible to test?

I'd appreciate any help!

Thanks