GerritUrlReader: Implemented "readTree"

"readTree" has been implemented for the "GerritUrlReader". Gerrit have
a REST API's to download repo contents but there are a number of
limitations that makes it unusable.

This implementation works as follows:
* The project and branch is parsed from the url.
* The current revision is fetched from the Gerrit REST API.
* The revision string is used as "etag".
* If the etag has changed a temporary directory is created.
* The project is cloned to the temporary directory.
* The cloned content is read into a "Readable Stream".
* The temporary directory is removed.
* "readTree" returns a response using "fromTarArchive" as read from the
  temporary directory.

Also added an option to specify the base "cloneUrl" has been added to the
gerrit integration config.

Signed-off-by: Niklas Aronsson <niklasar@axis.com>

packages/integration/config.d.ts

@@ -108,6 +108,11 @@ export interface Config {
        * @visibility frontend
        */
       baseUrl?: string;
+      /**
+       * The base url for cloning repos.
+       * @visibility frontend
+       */
+      cloneUrl?: string;
       /**
        * The username to use for authenticated requests.
        * @visibility secret