[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/enterprise/message_export/global_relay_export/global_relay_export.go

@@ -351,12 +351,12 @@ func addPostToChannelExport(rctx request.CTX, channelExport *ChannelExport, post
 	if err != nil {
 		rctx.Logger().Warn("Failed to unmarshal post Props into JSON. Ignoring username override.", mlog.Err(err))
 	} else {
-		if overrideUsername, ok := postPropsLocal["override_username"]; ok {
+		if overrideUsername, ok := postPropsLocal[model.PostPropsOverrideUsername]; ok {
 			postUserName = overrideUsername.(string)
 		}
 
 		if postUserName == originalUsername {
-			if overrideUsername, ok := postPropsLocal["webhook_display_name"]; ok {
+			if overrideUsername, ok := postPropsLocal[model.PostPropsWebhookDisplayName]; ok {
 				postUserName = overrideUsername.(string)
 			}
 		}