web_backend: added GetExternalJWT function

To support requesting external JWTs to use them as verification tokens.

src/web_service/web_backend.h

@@ -46,6 +46,13 @@ public:
     Common::WebResult DeleteJson(const std::string& path, const std::string& data,
                                  bool allow_anonymous);
 
+    /**
+     * Requests an external JWT for the specific audience provided.
+     * @param audience the audience of the JWT requested.
+     * @return the result of the request.
+     */
+    Common::WebResult GetExternalJWT(const std::string& audience);
+
 private:
     struct Impl;
     std::unique_ptr<Impl> impl;