Fix for parent-child state machine calls

[nplusterio]

Hello,

This PR allows adding -stepFunctionsEndpoint argument when starting Step Functions Local.

Background:

Been using serverless-step-functions-local and it's working well locally for individual state machine runs.
However, when running a step machine that calls another step machine as a task, I'm getting this error message:

{"Type":"ExecutionFailed","PreviousEventId":5,"ExecutionFailedEventDetails":{"Error":"StepFunctions-AWSStepFunctionsException","Cause":"The security token included in the request is invalid. (Service: AWSStepFunctions; Status Code: 400; Error Code: UnrecognizedClientException; Request ID: 72e40098-51a3-4a15-83b4-0d30b57c0f56; Proxy: null)"}}

(Note: --endpoint-url http://localhost:8083 was specified when running the aws stepfunctions command)

Adding the -stepFunctionsEndpoint argument with http://localhost:8083 as the value did the trick and executed the child state machines.

Next Step: Pass the stepFunctionsEndpoint from serverless-step-functions-local index.js#L96-L101