feat: move Linear config to installation_config (#10999)

This PR has the following changes

- Add `LINEAR_CLIENT_ID` and `LINEAR_CLIENT_SECRET` to installation
config
- Add Linear config to super_admin/features.yml
- Replace usage of ENV.fetch with GlobalConfigService.load for fetch
Client Id and Secret.

enterprise/app/helpers/super_admin/features.yml

@@ -79,3 +79,9 @@ microsoft:
   enabled: true
   icon: 'icon-microsoft'
   config_key: 'microsoft'
+linear:
+  name: 'Linear'
+  description: 'Configuration for setting up Linear'
+  enabled: true
+  icon: 'icon-linear'
+  config_key: 'linear'