Plan(s) listing in the `result` structure

I do not see a reason for exposing internal implementation details in the returned structure. If the planner finds multiple plans, they should be returned in a uniform structure such as

```json
{
"result": {
"plans": [
]
}
}
```

Currently I see `result.output.plan` as well as `result.output.sas_plan` or `result.output.sas_plan.1`. We are bleeding implementation details through the interface. Or I at least cannot see any added value here.

I am not discussing here whether the content of the `plans` array should be strings (as today) or some rich structure. But it should be an array capturing plans in order of discovery.