feat: support script sort

[cobbinma]

Hi

I have a query where I need to use a painless script sort but the current type of map[string]string isn't compatible with using complex sorts.

The issue is the json marshal will escape the sort string before marshalling it. Moving to map[string]interface will avoid escaping and enable us to use more complex types. The example below will work as intended:

	scriptSort := map[string]interface{}{
		"_script": map[string]interface{}{
			"type": "string",
			"script": map[string]interface{}{
				"source": "String name = doc['name'].value; return name.startsWith('name.prefix.') ? name.substring(12) : name;",
				"lang":   "painless",
			},
			"order": "asc",
		},
	}