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>

IMPLEMENTATION_COVERAGE.md

@@ -459,18 +459,18 @@
 
 ## application-autoscaling
 <details>
-<summary>0% implemented</summary>
+<summary>20% implemented</summary>
 
 - [ ] delete_scaling_policy
 - [ ] delete_scheduled_action
 - [ ] deregister_scalable_target
-- [ ] describe_scalable_targets
+- [x] describe_scalable_targets
 - [ ] describe_scaling_activities
 - [ ] describe_scaling_policies
 - [ ] describe_scheduled_actions
 - [ ] put_scaling_policy
 - [ ] put_scheduled_action
-- [ ] register_scalable_target
+- [x] register_scalable_target - includes enhanced validation support for ECS targets
 </details>
 
 ## application-insights