feat: show HMAC and enforcement for identity validation for API inbox (#4807)

Display HMAC token in Settings of Inbox API and enable enforcement option. I have also adjusted the copy so that it can apply to all inbox types.

Fixes: #4804

app/views/api/v1/models/_inbox.json.jbuilder

@@ -76,6 +76,7 @@ end
 
 ## API Channel Attributes
 if resource.api?
+  json.hmac_token resource.channel.try(:hmac_token)
   json.webhook_url resource.channel.try(:webhook_url)
   json.inbox_identifier resource.channel.try(:identifier)
   json.additional_attributes resource.channel.try(:additional_attributes)