Plugins: Support templated urls in routes (#16599)

This adds support for using templated/dynamic urls in routes.
* refactor interpolateString into utils and add interpolation support for app plugin routes.
* cleanup and add error check for url parse failure
* add docs for interpolated route urls

Closes #16835

(cherry picked from commit b07d0b1026)

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")
 	})