Add boolean similarity to built in similarity types

[brusic]

The new boolean similarity is not added to the builtin similarities, so trying to use it will fail:

PUT /test_sim_boolean
{
  "settings": {
    "index": {
      "similarity": {
        "default": {
          "type": "boolean"
        }
      }
    }
  }
}

PUT test_sim_boolean
{
  "settings": {
    "similarity": {
      "boolean_sim": {
        "type": "boolean"
      }
    }
  }
}

{
   "error": {
      "root_cause": [
         {
            "type": "illegal_argument_exception",
            "reason": "Unknown Similarity type [boolean] for [default]"
         }
      ],
      "type": "illegal_argument_exception",
      "reason": "Unknown Similarity type [boolean] for [default]"
   },
   "status": 400
}

The workaround is to simply use the similarity directly in every field that requires it

PUT /test_sim_boolean
{
    "mappings": {
      "foo": {
        "properties": {
          "bar": {
            "type": "text",
            "similarity": "boolean"
          }
        }
      }
    }
  }
}

• First commit contains the fix.
• Second commit is a simple rename of the local variable. I can remove this commit if you really like the previous name or I can squash it.
• Third commit: after analyzing the code in order to future proof it, it is clear based on the three data points that built-in similarities include all of the default ones

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[jpountz]

Good catch, the change looks good to me.

[jpountz]

@elasticmachine please test it