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/utils.go

@@ -0,0 +1,49 @@
+package pluginproxy
+
+import (
+	"bytes"
+	"fmt"
+	"net/url"
+	"text/template"
+
+	m "github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/plugins"
+)
+
+// InterpolateString accepts template data and return a string with substitutions
+func InterpolateString(text string, data templateData) (string, error) {
+	t, err := template.New("content").Parse(text)
+	if err != nil {
+		return "", fmt.Errorf("could not parse template %s", text)
+	}
+
+	var contentBuf bytes.Buffer
+	err = t.Execute(&contentBuf, data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute template %s", text)
+	}
+
+	return contentBuf.String(), nil
+}
+
+// InterpolateURL accepts template data and return a string with substitutions
+func InterpolateURL(anURL *url.URL, route *plugins.AppPluginRoute, orgID int64, appID string) (*url.URL, error) {
+	query := m.GetPluginSettingByIdQuery{OrgId: orgID, PluginId: appID}
+	result, err := url.Parse(anURL.String())
+	if query.Result != nil {
+		if len(query.Result.JsonData) > 0 {
+			data := templateData{
+				JsonData: query.Result.JsonData,
+			}
+			interpolatedResult, err := InterpolateString(anURL.String(), data)
+			if err == nil {
+				result, err = url.Parse(interpolatedResult)
+				if err != nil {
+					return nil, fmt.Errorf("Error parsing plugin route url %v", err)
+				}
+			}
+		}
+	}
+
+	return result, err
+}