Add text input field to trigger_field.vue

What does this MR do and why?

Adds an extra text input field to each trigger for an integration:

• This is used to specify which channel should the message be posted on for each trigger.
• The field will only show when the trigger is selected.

Note: This is not a user-facing change (yet) since the backend that defines the sections will be added separately.

Screenshots or screen recordings

Old UI (without sections)	New UI (with sections, without field)	New UI (with sections, with field)

How to set up and validate locally

1. Apply this patch to add sections to the GitLab for Slack app integration.

diff --git a/ee/app/helpers/ee/integrations_helper.rb b/ee/app/helpers/ee/integrations_helper.rb
index 5479bea1c883..39b5115ecec0 100644
--- a/ee/app/helpers/ee/integrations_helper.rb
+++ b/ee/app/helpers/ee/integrations_helper.rb
@@ -26,7 +26,7 @@ def integration_form_data(integration, project: nil, group: nil)
 
       if integration.is_a?(::Integrations::GitlabSlackApplication)
         form_data[:upgrade_slack_url] = add_to_slack_link(project, slack_app_id)
-        form_data[:should_upgrade_slack] = integration.slack_integration.upgrade_needed? ? 'true' : 'false'
+        # form_data[:should_upgrade_slack] = integration.slack_integration.upgrade_needed? ? 'true' : 'false'
       end
 
       form_data
diff --git a/ee/app/models/integrations/gitlab_slack_application.rb b/ee/app/models/integrations/gitlab_slack_application.rb
index faf86cfa2511..6dcc972d268e 100644
--- a/ee/app/models/integrations/gitlab_slack_application.rb
+++ b/ee/app/models/integrations/gitlab_slack_application.rb
@@ -53,6 +53,7 @@ def testable?
     # flow has been completed, which causes the integration to become activated/enabled.
     override :editable?
     def editable?
+      return true
       activated? && Feature.enabled?(:integration_slack_app_notifications, project)
     end
 
@@ -63,6 +64,22 @@ def fields
       super
     end
 
+    override :sections
+    def sections
+      [
+        {
+          type: SECTION_TYPE_TRIGGER,
+          title: s_('Integrations|Trigger'),
+          description: s_('Integrations|An event will be triggered when one of the following items happen.')
+        },
+        {
+          type: SECTION_TYPE_CONFIGURATION,
+          title: s_('Integrations|Notification settings'),
+          description: s_('Integrations|Configure the scope of notifications.')
+        }
+      ]
+    end
+
     override :configurable_events
     def configurable_events
       return [] unless editable?

2. Go to Project > Settings > Integrations > GitLab for Slack app. For example, http://127.0.0.1:3000/gitlab-org/gitlab-test/-/settings/integrations/gitlab_slack_application/edit.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #384886 (closed)