Search configuration data

Created by: sqs

A request to be able to see search results in files that do not exist in a Git repository, such as configuration files served by a system like etcd/Zookeeper. This would be provided by a Sourcegraph extension using the search result provider API. It would eventually run on the backend but TBD on whether that would be in scope for 3.1 (highly unlikely running it on the backend too would be fully ready, it might be feature-flagged off by default).