[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/public/pluginapi/post.go

@@ -297,7 +297,7 @@ func (p *PostService) ShouldProcessMessage(post *model.Post, options ...ShouldPr
 		return false, nil
 	}
 
-	if !messageProcessOptions.AllowWebhook && post.GetProp("from_webhook") == "true" {
+	if !messageProcessOptions.AllowWebhook && post.GetProp(model.PostPropsFromWebhook) == "true" {
 		return false, nil
 	}