[MM-62427] Add message attachments validation (#30180)

* Add message attachments validation

* Add props validation

* Validate slack attachment fields

* Update tests and library usage

* Improve interactive dialog error for length checks

* Allow predefined colors for slack attachments

* Fix TestPostAction

* Use const for data source

* Add tests

* Cleanup unused props

* Add happy path tests

* lint fixes

* Add validation for PostActionOptions

server/channels/app/notification_push_test.go

@@ -1617,7 +1617,7 @@ func TestPushNotificationAttachment(t *testing.T) {
 	post := &model.Post{
 		Message: originalMessage,
 		Props: map[string]any{
-			"attachments": []*model.SlackAttachment{
+			model.PostPropsAttachments: []*model.SlackAttachment{
 				{
 					AuthorName: "testuser",
 					Text:       "test attachment",