New Sample - Cross-execution concurrency control for Step Function workflow

[junguo]

Map state can be used to control concurrency within the same execution, however there is no built-in mechanism to control cross-execution concurrency. There are actual use cases where one wants to cap the number of executions in a workflow due to limitation such as API invocation quota and/or human involvement. This project is to develop a sample flow/pattern that utilizes listExecutions() API call to implement a cross-execution concurrency control over step function work flows