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.

app/helpers/linear/integration_helper.rb

@@ -32,7 +32,7 @@ module Linear::IntegrationHelper
   private
 
   def client_secret
-    @client_secret ||= ENV.fetch('LINEAR_CLIENT_SECRET', nil)
+    @client_secret ||= GlobalConfigService.load('LINEAR_CLIENT_SECRET', nil)
   end
 
   def decode_token(token, secret)