[MM-11860]: Expose slack attachment parsing functions in the model package (#9351)

Refactored parseSlackAttachment functions from https://github.com/mattermost/mattermost-server/blob/master/app/post.go#L312
into model/slack_attachments.go so that plugins have access to them.

app/webhook.go

@@ -265,7 +265,7 @@ func (a *App) CreateWebhookPost(userId string, channel *model.Channel, text, ove
 		for key, val := range props {
 			if key == "attachments" {
 				if attachments, success := val.([]*model.SlackAttachment); success {
-					parseSlackAttachment(post, attachments)
+					model.ParseSlackAttachment(post, attachments)
 				}
 			} else if key != "override_icon_url" && key != "override_username" && key != "from_webhook" {
 				post.AddProp(key, val)