Elastic: Support request cancellation properly (Uses new backendSrv.fetch Observable request API) (#30009)

* Elastic: Fixes so templating queries work

* Chore: fixes test

* Fix: fixes getFields from metricFindQuery

* Elastic: Support request cancellation properly

* Refactor: refactors tests

Co-authored-by: Elfo404 <gio.ricci@grafana.com>