[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/plugin_api.go

@@ -756,7 +756,7 @@ func (api *PluginAPI) DeleteGroupSyncable(groupID string, syncableID string, syn
 }
 
 func (api *PluginAPI) CreatePost(post *model.Post) (*model.Post, *model.AppError) {
-	post.AddProp("from_plugin", "true")
+	post.AddProp(model.PostPropsFromPlugin, "true")
 
 	post, appErr := api.app.CreatePostMissingChannel(api.ctx, post, true, true)
 	if post != nil {