As an enhancement to https://github.com/localstack/localstack/pull/1194

It would be really helpful to send the Lambda name, either as data, a header, or as part of the URL.

At code location: https://github.com/localstack/localstack/blob/03c6f4bf250edd18eb818ed65090f508636b0bff/localstack/services/awslambda/lambda_api.py#L696

I can likely open a PR to handle this given some feedback on the feature and potential design.

Right now all I see is the following (using `npx http-echo-server`):
```
[socket#8] event: data
--> POST / HTTP/1.1
--> Host: 192.168.1.x:3000
--> User-Agent: python-requests/2.22.0
--> Accept-Encoding: gzip, deflate
--> Accept: */*
--> Connection: keep-alive
--> Content-Length: 9
--> Content-Type: application/x-www-form-urlencoded
-->
-->
[socket#8] event: data
--> Name=test
```
runtime=LAMBDA_RUNTIME_PYTHON36)
        state_machine_def = {
            'StartAt': 'step1',
            'States': {
                'step1': {
                    'Type': 'Task',
                    'Resource': aws_stack.lambda_function_arn(resource_lambda_name),
                    'ResultPath': '$.result_value',
                    'End': True
                }
            }
        }
        sfn_client = aws_stack.connect_to_service('stepfunctions')
        rs = sfn_client.create_state_machine(
            name=state_machine_name,
            definition=json.dumps(state_machine_def),
            roleArn=aws_stack.role_arn('sfn_role')
        )
        sm_arn = rs['stateMachineArn']
        self.lambda_client.invoke(
            FunctionName=function_name,
            Payload=json.dumps({
                'state_machine_arn': sm_arn,
                'region_name': config.DEFAULT_REGION,
                'input': {}
            })
        )
        time.sleep(1)
        rs = sfn_client.list_executions(
            stateMachineArn=sm_arn