This change introduces a new API called `_field_caps` that allows to retrieve the capabilities of specific fields.
This field centric API relies solely on the mapping of the requested indices to extract the following infos:
* `types`: one or many field types if the type is not the same across the requested indices
* `searchable`: Whether this field is indexed for search on at least one requested indices.
* `aggregatable`:  Whether this field can be aggregated on at least one requested indices.

Example:

````
GET t,v/_field_caps?fields=field1,field2,field3
````

returns:

````
{
   "fields": {
      "field1": {
         "_all": {
            "types": "text",
            "searchable": true,
            "aggregatable": false
         }
      },
      "field3": {
         "_all": {
            "types": "text",
            "searchable": true,
            "aggregatable": false
         }
      },
      "field2": {
         "_all": {
            "types": [
               "keyword",
               "long"
            ],
            "searchable": true,
            "aggregatable": true
         }
      }
   }
}
````

In this example `field1` and `field3` have the same type `text` across the requested indices `t` and `v`.
Conversely `field2` is defined with two conflicting types `keyword` and `long`.
Note that `_field_caps` does not treat this case as an error but rather return the list of unique types seen for this field.

It is also possible to get a view of each index field capabilities with the `level` parameter:

````
GET t,v/_field_caps?fields=field2&level=indices
````

````
{
   "fields": {
      "field2": {
         "t": {
            "types": "keyword",
            "searchable": true,
            "aggregatable": true
         },
         "v": {
            "types": "long",
            "searchable": true,
            "aggregatable": true
         }
      }
   }
}
````