chore: Add feature flag for conversation continuity in API (#5399)

- Add the ability to toggle conversation continuity in API channel

app/models/message.rb

@@ -202,8 +202,12 @@ class Message < ApplicationRecord
     inbox.web_widget? && inbox.channel.continuity_via_email
   end
 
+  def email_notifiable_api_channel?
+    inbox.api? && inbox.account.feature_enabled?('email_continuity_on_api_channel')
+  end
+
   def email_notifiable_channel?
-    email_notifiable_webwidget? || %w[Email].include?(inbox.inbox_type)
+    email_notifiable_webwidget? || %w[Email].include?(inbox.inbox_type) || email_notifiable_api_channel?
   end
 
   def can_notify_via_mail?

config/features.yml

@@ -11,3 +11,5 @@
   enabled: false
 - name: disable_branding
   enabled: false
+- name: email_continuity_on_api_channel
+  enabled: false