* extra data source read methods * update tests * more tests * fix more tests; actually initialize retriever instead of sending nil * moving GetAllDataSources isn't strictly required, so keep to minimal changes * better name for retriever logger Co-authored-by: Dafydd <72009875+dafydd-t@users.noreply.github.com> * add compile-time check for DS retriever impl --------- Co-authored-by: Dafydd <72009875+dafydd-t@users.noreply.github.com> Co-authored-by: Stephanie Hingtgen <stephanie.hingtgen@grafana.com>