feat: explicitly add filter:admin/header.build hook

As it is not fired during middleware.processRender

src/middleware/admin.js

@@ -79,6 +79,9 @@ middleware.renderHeader = async (req, res, data) => {
 	templateValues.template = { name: res.locals.template };
 	templateValues.template[res.locals.template] = true;
 
+	// Normally this should hook be automatically added by middleware.processRender(), but it seems to only be fired for page hooks, and not when called internally.
+	({ templateValues } = await plugins.hooks.fire('filter:admin/header.build', { req, res, templateData: templateValues }));
+
 	return await req.app.renderAsync('admin/header', templateValues);
 };