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.

config/installation_config.yml

@@ -243,3 +243,16 @@
   locked: false
   description: 'Contents on your firebase credentials json file'
 ## ------ End of Configs added for FCM v1 notifications ------ ##
+
+## ------ Configs added for Linear ------ ##
+- name: LINEAR_CLIENT_ID
+  display_title: 'Linear Client ID'
+  value:
+  locked: false
+  description: 'Linear client ID'
+- name: LINEAR_CLIENT_SECRET
+  display_title: 'Linear Client Secret'
+  value:
+  locked: false
+  description: 'Linear client secret'
+## ------ End of Configs added for Linear ------ ##