[ML] Add custom all field to config index

[davidkyle]

This would enable search/filtering of ml job configurations by key fields in the config, for example if all the fields used by detectors were copied to a custom all field then it would be simple to find all jobs that use a particular field.

Candidate Job config fields to copy to an custom all field:

• Detector fields
• Detector function
• Bucket span
• The job groups
• The job Id

For Datafeeds:

• ?

Pinging @elastic/ml-ui for input on how such a feature could be used.

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

We also need to think how/if this would work with the new analytics configs. We shouldn't tie the .ml-config index so closely to anomaly detector configs that it cannot be used for other types of configs too.

[davidkyle]

Even with just datafeeds and jobs we could hit the problem where a search returns both types if they happen to contain the value searched for. In this case the UI could filter out the config types it is not interested in i.e. the jobs page. Even if a filter has to be applied to the config type is still enables missing functionality.

{
  "query": {
    "bool" : {
      "must" : {
        "term" : { "config_type" : "anomaly_detector" }
      },
      "must": {
        "match": {
          "custom_all_field": { 
            "query": "airline"
          }
        }
      }  
    }
  }
}

[droberts195]

.ml-config is now a system index and cannot be searched directly, so this is no longer relevant.