Issue 6552 - Support arbitrary manifest values

templates/manifest.json.tmpl

 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 {
   "manifest_version": 2,
   {%- if releaseBuild %}
   "name": "__MSG_name__",
   {%- else %}
   "name": "__MSG_name_devbuild__",
@@ skipping 93 matching lines @@
   {%- endif %}
 
   {%- if contentScripts %}
   "content_scripts": {{contentScripts|json}},
   {%- endif %}
 
   {%- if webAccessible %}
   "web_accessible_resources": {{webAccessible|json}},
   {%- endif %}
 
+  {%- if externallyConnectable %}
+    "externally_connectable": {
+      {% for key in externallyConnectable %}
+        "{{key}}": {{externallyConnectable[key]|json}}{%- if not loop.last -%},{%- endif -%}
+      {% endfor %}
+    },
+  {%- endif %}
+
   {%- if metadata.has_option('general', 'managedStorageSchema') %}
   "storage": {
     "managed_schema": {{metadata.get('general', 'managedStorageSchema')|json}}
   },
   {%- endif %}
 
   {%- if metadata.has_option('general', 'contentSecurityPolicy') %}
   "content_security_policy": {{metadata.get('general', 'contentSecurityPolicy')|json}},
   {%- endif %}
 
   "_dummy": false
 }