DSL class should be accessible from the Controller

### Description

Since as of #42, we have the ability to run custom queries, the DSL class should also be accessible to aid in the creation of these queries. Without it, the queries become dependent on the specific service in use, which while this is _OK_, it is as optimized as it could be. If the DSL class was accessible from the controller, then some of the service-specific DSL and field mappings would be available for use in custom queries, making the custom queries used by theme/plugin code sturdier, more future-proof, and more standardized.

### Use Case

When writing custom DSL for custom queries, a developer should be able to utilize Elasticsearch-Extension's DSL class so that they can leverage adapter-agnostic DSL and field mappings.