GetAutoscalingPolicy API should have an option to return deciders enabled by default

[ywangd]

The GetAutoscalingPolicy API does not include deciders enabled by default for a policy. This is rather confusing since it seems to suggest that no decider is configured. But you can check the actual effective deciders with the GetAutoscalingCapacity API which does include deciders enabled by default. We should consider adding an option to allow default deciders for each policy to be returned in the response for GetAutoscalingPolicy API.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)