Search Templates: Add render endpoint for rendering templates

This commit adds a render endpoint to allow rendering of templates.
````
POST /_render/template
'{
  "template":
  {
    "query":
      {
        "{{foo}}": {}
      },
    "size": "{{my_{{}}size}}"
   },
   "params": { "foo": "match_all", "my_size": 10}
}'
````
Will render
````
{
  "template" :
  {
    "query" : {
      "match_all" : {}
    },
    "size" : 10
  }
}
````

Closes #6821