* Implement filtering for stepfunctions:ListExecutions
* Add pagination to Step Functions endpoints
Implements a generalized approach to pagination via a decorator method for the
Step Functions endpoints. Modeled on the real AWS backend behavior, `nextToken`
is a dictionary of pagination information encoded in an opaque string.
With just a bit of metadata hard-coded (`utils.PAGINATION_MODEL`), backend `list`
methods need only be decorated with `@paginate` and ensure that their returned
entities are sorted to get full pagination support without any duplicated code
polluting the model.
Closes#3137
* Add check for existing execution, fix issue with make init
* Remove f-string usage
* Remove fstring usage in test
* Pin black and run formatting on test_stepfunction
* Reverse changes made by black 20.8b1
