Chore: Improve types (#85659)

* fixing some types

* more type fixes

* few more

* last couple

* tiny tweak

public/app/plugins/datasource/grafana-postgresql-datasource/datasource.test.ts

@@ -812,6 +812,6 @@ const createFetchResponse = <T>(data: T): FetchResponse<T> => ({
   type: 'basic',
   statusText: 'Ok',
   redirected: false,
-  headers: {} as unknown as Headers,
+  headers: new Headers(),
   ok: true,
 });