Add external_tracker to JiraIssueSerializer

Needed by frontend to differentiate between Jira issues
and normal issues.

ee/app/serializers/integrations/jira/issue_entity.rb

@@ -64,6 +64,10 @@ module Integrations
           relative: jira_issue.key
         }
       end
+
+      expose :external_tracker do |_jira_issue|
+        'jira'
+      end
     end
   end
 end

ee/spec/serializers/integrations/jira/issue_entity_spec.rb

@@ -40,7 +40,8 @@ RSpec.describe Integrations::Jira::IssueEntity do
         { name: 'assignee' }
       ],
       web_url: 'http://jira.com/projects/GL/issues/GL-5',
-      references: { relative: 'GL-5' }
+      references: { relative: 'GL-5' },
+      external_tracker: 'jira'
     )
   end