Application Autoscaling basic features (#3082)

* Placeholder to test Application Autoscaling.

* Wire everything together and create a first passing test without any real functionality.

* Get one test working properly.

* Add some TODO items.

* Reformat code with black

* Second passing test for describe_scalable_targets.

* New test for NextToken.

* Add some tests for ParamValidationError and ValidationException.

* black

* Ensure scalable targets are being captured in an OrderedDict() for deterministic return later.

* Add validation to describe_scalable_targets and register_scalable_target.

* Fix tests.

* Add creation_time, refactor, add ECS backend, and add failing test for checking that ecs service exists.

* Add parameter validation.

* Improved documentation for CONTRIBUTING.md

Adds some details to give people an idea what's involved in adding new features/services

* Integrate with ECS.

* black

* Refactor to allow implementation of SuspendedState.

* Complete support for SuspendedState.

* Bump up implementation coverage percentage.

* Tidy up code; add comments.

* Implement suggested changes from code review.

* Minor refactorings for elegance.

* README update

Co-authored-by: Bert Blommers <bblommers@users.noreply.github.com>

moto/backends.py

@@ -6,6 +6,10 @@ BACKENDS = {
     "acm": ("acm", "acm_backends"),
     "apigateway": ("apigateway", "apigateway_backends"),
     "athena": ("athena", "athena_backends"),
+    "applicationautoscaling": (
+        "applicationautoscaling",
+        "applicationautoscaling_backends",
+    ),
     "autoscaling": ("autoscaling", "autoscaling_backends"),
     "batch": ("batch", "batch_backends"),
     "cloudformation": ("cloudformation", "cloudformation_backends"),