Alerting: Add filtering for Silences (#39109)

* Add filtering for Silences page

* Add tests Silences and SilenceEditor

* pr feedback: add field validation and test refactor

* Add test for checking content

* fix overflow for validation error message

* increase login threshold for pa11y

* Make silence filter state its own type and function

public/app/types/unified-alerting.ts

@@ -133,4 +133,10 @@ export interface FilterState {
   dataSource?: string;
   alertState?: string;
   groupBy?: string[];
+  silenceState?: string;
+}
+
+export interface SilenceFilterState {
+  queryString?: string;
+  silenceState?: string;
 }