[MM-29845] Add CWS Webhook endpoint and payment failed email (#16351)

* Add a new handler to allow authentication via CWS API Key

* Make error better

* Add tests and cases for new handler functions

* Move some code around

* Add test for GetCloudSession function

* unset the env after test completion

* Remove white space

* Add CWS Webhook endpoint and email code

* handle returned errors from email sending function

* Change FailureCode to FailureMessage

* Remove unnecessary translations

* Fix translations

* Forgot to add template

* Update api4/cloud.go

Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>

* Update api4/cloud.go

Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>

* Update api4/cloud.go

Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>

* PR changes

* Update app/email.go

Co-authored-by: Mario de Frutos Dieguez <mario@defrutos.org>

* Close body in proper spot

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>
Co-authored-by: Mario de Frutos Dieguez <mario@defrutos.org>

model/cloud.go

@@ -3,6 +3,10 @@
 
 package model
 
+const (
+	EventTypeFailedPayment = "failed-payment"
+)
+
 // Product model represents a product on the cloud system.
 type Product struct {
 	ID           string   `json:"id"`
@@ -112,3 +116,14 @@ type InvoiceLineItem struct {
 	Type         string                 `json:"type"`
 	Metadata     map[string]interface{} `json:"metadata"`
 }
+
+type CWSWebhookPayload struct {
+	Event         string         `json:"event"`
+	FailedPayment *FailedPayment `json:"failed_payment"`
+}
+
+type FailedPayment struct {
+	CardBrand      string `json:"card_brand"`
+	LastFour       int    `json:"last_four"`
+	FailureMessage string `json:"failure_message"`
+}