Add post priority to incoming and outgoing webhook (#26671)

* Added priority to incoming and outgoing webhook

* fixed fat finger

* Added priority tests

* fixed golint

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/outgoing_webhook.go

@@ -73,6 +73,7 @@ type OutgoingWebhookResponse struct {
 	Attachments  []*SlackAttachment `json:"attachments"`
 	Type         string             `json:"type"`
 	ResponseType string             `json:"response_type"`
+	Priority     *PostPriority      `json:"priority"`
 }
 
 const OutgoingHookResponseTypeComment = "comment"