Create generic framework for auto complete

Add an easy way to support auto complete for any endpoint.

Initially focus on ES, but also leave room for WP.com public api, and for the new WP.org REST API.

Thoughts:
- a bunch of .json files that can be included in Inquire
- Also allow loading .json from an endpoint so a particular api can keep the auto suggest up to date.
- How do we handle versioning?

No files or tests are named. Start by reviewing Inquire’s existing endpoint handling and the Elasticsearch autocomplete path, then compare the proposed bundled or endpoint-loaded JSON with the WordPress API targets. Done requires an agreed generic design covering endpoint loading, versioning, and initial Elasticsearch support.

Written by the indexing model from the issue text.