Normalize the submit task API to be more like the supervisor API

[vogievetsky]

Description

Right now submitting a task expects a payload like:

{
  "type": "index",
  "spec": {
    "dataSchema": { ... },
    "tuningConfig": { ... },
    "ioConfig": { ... }
  }
}

(see https://druid.apache.org/docs/latest/tutorials/tutorial-batch.html#loading-data-with-a-spec-via-console)

While a supervisor has a payload of

{
  "type": "kafka",
  "dataSchema": { ... },
  "tuningConfig": { ... },
  "ioConfig": { ... }
}

(see https://druid.apache.org/docs/latest/tutorials/tutorial-kafka.html)

This is a hastle for the console to handle and also confusing to users ( https://stackoverflow.com/questions/58097806/cant-create-druid-ingestion-task-through-api/58131519#58131519 )

I suggest to normalize the task API to be able to accept

{
  "type": "index",
  "dataSchema": { ... },
  "tuningConfig": { ... },
  "ioConfig": { ... }
}

as the payload. It should also be backwards compatible.

Implementation wise it could just rewrite it to the spec form internally.