Application Autoscaling basic features

[dawngerpony]

Fixes #1931.

Problem

moto currently has no support for mocking any of the Application Load Balancing features of boto3. See #1931 for more information.

Changes

Current status of this PR:

• basic support for describe_scalable_targets with "consistency validation" for ECS-based targets
• "consistency validation" (my own term) means that you must mock ECS and create a service before you can register any scalable targets against that service, i.e. your scalable targets must be consistent with the mock resources to which they refer
• basic support for register_scalable_target
• parameter validation over and above the basic validation provided by the boto3 client
• README and contributor guide updates

TODO

• More comprehensive parameter validation
• support for additional endpoints (should be in another PR)
• better support (e.g. "consistency validation") for additional services supported by Application Autoscaling, similar to that already implemented for ECS in this PR. The full list seems to be:

    APPSTREAM = "appstream"
    RDS = "rds"
    LAMBDA = "lambda"
    CASSANDRA = "cassandra"
    DYNAMODB = "dynamodb"
    CUSTOM_RESOURCE = "custom-resource"
    ELASTICMAPREDUCE = "elasticmapreduce"
    EC2 = "ec2"
    COMPREHEND = "comprehend"
    ECS = "ecs"
    SAGEMAKER = "sagemaker"

Notes

• The changes in this PR were inspired by Add managedblockchain network functions #2956 #2957 as well as the existing autoscaling functionality, which is far closer to the functionality required by Application Autoscaling than the Managed Blockchain functionality.
• This is my first PR implementing an entirely new endpoint; feedback is welcomed 🙏

[coveralls]

Coverage decreased (-0.02%) to 94.378% when pulling b65c59c on dawngerpony:feature/1931-application-autoscaling into 385c78a on spulec:master.

[dawngerpony]

NB. Build is currently failing due to #3098

[bblommers]

LGTM. Thanks for your contribution @dawngerpony!

I'm going to have to leave this open for now - will merge it after we get master fixed

[dawngerpony]

LGTM. Thanks for your contribution @dawngerpony!

I'm going to have to leave this open for now - will merge it after we get master fixed

No problem, I've tried merging your changes from #3099 into my branch to see if I can at least get a green build while I'm waiting for master to go green.

[dawngerpony]

@bblommers I think this PR might be ready to merge now? I've pushed your build fix from master into my branch.