Add support for actions in the unreads bar from plugins (#24265)

* Add support for actions in the unreads bar from plugins

* Adding channelId as parameter

* Fixing linter errors

* Changing the extensibility to the new messages separator

* Making everything work with the plugin

* Fixing linter and types errors

* Fixing unit test

* Tiny improvement in the styles

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/plugins/registry.ts

@@ -508,6 +508,12 @@ export default class PluginRegistry {
         return dispatchPluginComponentAction('PostEditorAction', this.id, component);
     });
 
+    // Register a component to the add to the new messages separator.
+    // Accepts a React component. Returns a unique identifier.
+    registerNewMessagesSeparatorActionComponent = reArg(['component'], ({component}: DPluginComponentProp) => {
+        return dispatchPluginComponentAction('NewMessagesSeparatorAction', this.id, component);
+    });
+
     // Register a post menu list item by providing some text and an action function.
     // Accepts the following:
     // - text - A string or React element to display in the menu