### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Currently, when passing `$$.Execution.StartTime` as a parameter to a SFN lambda, it is coming in in the format `HH:MM:SS.###`.

### Expected Behavior

`$$.Execution.StartTime` should be an ISO 8601 datetime string.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

This is run in a docker compose file.

Step function definition is

```json
{
  "Comment": "Sample SFN",
  "StartAt": "SampleLambda",
  "States": {    
    "SampleLambda": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "End": true,
      "Parameters": {
        "FunctionName": "Sample-Function",
        "Payload": {
          "data": {
            "startDateTime.$": "$$.Execution.StartTime"
          }
        }
      }
    }
  }
}
```

### Environment

```markdown
- OS: Mac OS Ventura 13.6.1
- LocalStack:
LocalStack version: 3.0.1.dev
LocalStack build date: 2023-11-20
LocalStack build git hash: 3e32b438
```


### Anything else?

Thank you so much for your hard work.
if self.exec_worker:
            raise InvalidName()  
        self.exec_worker = ExecutionWorker(
            definition=self.state_machine.definition,
            input_data=self.input_data,
            exec_comm=BaseExecutionWorkerComm(self),
            context_object_init=ContextObjectInitData(
                Execution=ContextObjectExecution(
                    Id=self.exec_arn,
                    Input=self.input_data,
                    Name=self.name,
                    RoleArn=self.role_arn,
                    StartTime=self.start_date.time().isoformat(),
                ),
                StateMachine=ContextObjectStateMachine(
                    Id=self.state_machine.arn,
                    Name=self.state_machine.name,
                ),
            ),
            aws_execution_details=AWSExecutionDetails(
                account=self.account_id, region=self.region_name, role_arn=self.role_arn
            ),
        )
        self.exec_status = ExecutionStatus.RUNNING
        self._publish_execution_status_change_event()
        self.exec_worker.start()
    def stop(self, stop_date: datetime.datetime, error: Optional[str], cause: Optional[str]):