Add app_manifest to fetch manifest of an app from the catalog or using url

data/actionsmap/yunohost.yml

@@ -608,6 +608,14 @@ app:
                 string:
                     help: Return matching app name or description with "string"
 
+        ### app_manifest()
+        manifest:
+            action_help: Return the manifest of a given app from the manifest, or from a remote git repo
+            api: GET /apps/manifest
+            arguments:
+                app:
+                    help: Name, local path or git URL of the app to fetch the manifest of
+
         fetchlist:
             deprecated: true
 
@@ -679,7 +687,7 @@ app:
                     help: Do not ask confirmation if the app is not safe to use (low quality, experimental or 3rd party)
                     action: store_true
 
-        ### app_remove() TODO: Write help
+        ### app_remove()
         remove:
             action_help: Remove app
             api: DELETE /apps/<app>

src/yunohost/app.py

@@ -786,6 +786,21 @@ def app_upgrade(app=[], url=None, file=None, force=False):
     logger.success(m18n.n("upgrade_complete"))
 
 
+def app_manifest(app):
+
+    raw_app_list = _load_apps_catalog()["apps"]
+
+    if app in raw_app_list or ("@" in app) or ("http://" in app) or ("https://" in app):
+        manifest, extracted_app_folder = _fetch_app_from_git(app)
+    elif os.path.exists(app):
+        manifest, extracted_app_folder = _extract_app_from_file(app)
+    else:
+        raise YunohostValidationError("app_unknown")
+
+    shutil.rmtree(extracted_app_folder)
+
+    return manifest
+
 @is_unit_operation()
 def app_install(
     operation_logger,