feat: HMAC verification for web widget (#1643)

* feat: HMAC verification for web widget. Let you verify the authenticated contact via HMAC on the web widget to prevent data tampering.
* Add docs for identity-validation

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

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

@@ -17,3 +17,4 @@ json.phone_number resource.channel.try(:phone_number)
 json.selected_feature_flags resource.channel.try(:selected_feature_flags)
 json.reply_time resource.channel.try(:reply_time)
 json.reauthorization_required resource.channel.try(:reauthorization_required?) if resource.facebook?
+json.hmac_token resource.channel.try(:hmac_token) if resource.web_widget?