Added ecs.TaskDefinition.RuntimePlatform

[JohnPreston]

Added ecs.TaskDefinition.RuntimePlatform as per

• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-taskdefinition.html#cfn-ecs-taskdefinition-runtimeplatform
• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ecs-taskdefinition-runtimeplatform.html

[JohnPreston]

@markpeek any objections to putting the link to the property in the docstrings of the classes ?

[markpeek]

Thanks @JohnPreston. The link looks fine for reference purposes. I should add that to the generator I use.

[JohnPreston]

Thanks @markpeek