Explore: Adds support for new loki 'start' and 'end' params for labels endpoint (#17512)

* Explore: Adds support for new loki 'start' and 'end' params for labels endpoint
Also initializes absoluteRange when explore is initialized
Closes #16788

* Explore: Dispatches updateTimeRangeAction instead of passing absoluteRange when initializing
Also removes dependency on sinon from loki language provider test

* Loki: Refactors transformation of absolute time range to URL params into small utility function

* Makes use of rangeToParams() util function in loki language provider test
Also updates LanguageProvider.request() interface so that url should be type string, and adds optional params argument

public/app/plugins/datasource/loki/components/LokiQueryField.tsx

@@ -9,7 +9,8 @@ const LokiQueryField: FunctionComponent<LokiQueryFieldFormProps> = ({
 }) => {
   const { isSyntaxReady, setActiveOption, refreshLabels, ...syntaxProps } = useLokiSyntax(
     datasource.languageProvider,
-    datasourceStatus
+    datasourceStatus,
+    otherProps.absoluteRange
   );
 
   return (