Fix panic when using complex dynamic URLs in app plugin routes (#27977)

* remove unused function to interpolate URLs

* share function to add headers between ds/plugin proxies

* stop performing unnecessary plugin setting lookup

* fix bug causing runtime errors when using complex templated URLs

* lower case util functions not used outside of pluginproxy package

* change test URL to a (valid) dummy URL to make intent clearer

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/api/pluginproxy/ds_auth_provider_test.go

@@ -14,7 +14,7 @@ func TestDsAuthProvider(t *testing.T) {
 			},
 		}
 
-		interpolated, err := InterpolateString("{{.SecureJsonData.Test}}", data)
+		interpolated, err := interpolateString("{{.SecureJsonData.Test}}", data)
 		So(err, ShouldBeNil)
 		So(interpolated, ShouldEqual, "0asd+asd")
 	})