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/incoming_webhook.go

@@ -56,6 +56,7 @@ type IncomingWebhookRequest struct {
 	Attachments []*SlackAttachment `json:"attachments"`
 	Type        string             `json:"type"`
 	IconEmoji   string             `json:"icon_emoji"`
+	Priority    *PostPriority      `json:"priority"`
 }
 
 func (o *IncomingWebhook) IsValid() *AppError {