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

pkg/api/pluginproxy/utils_test.go

@@ -0,0 +1,21 @@
+package pluginproxy
+
+import (
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestInterpolateString(t *testing.T) {
+	Convey("When interpolating string", t, func() {
+		data := templateData{
+			SecureJsonData: map[string]string{
+				"Test": "0asd+asd",
+			},
+		}
+
+		interpolated, err := InterpolateString("{{.SecureJsonData.Test}}", data)
+		So(err, ShouldBeNil)
+		So(interpolated, ShouldEqual, "0asd+asd")
+	})
+}