New composer script handler to deploy SimpleSAMLphp assets

[mingsong-hu]

Instead of enabling the composer-module-installer plugin, this PR adds a custom Composer script handler (installSimpleSamlAssets) that:

1. Keeps "simplesamlphp/composer-module-installer": false — no other SSP modules can auto-install
2. Copies only the web-asset directories (css, js, icons, webfonts) from simplesamlphp-assets-base module into the asset folder required by a HTTP-POST binding login form.
3. Ignores non-asset files (.gitignore, .gitattributes, composer.json)
4. Runs automatically on composer install and composer update via post-install-cmd / post-update-cmd
5. Uses the existing ScriptHandler.php pattern and Symfony\Component\Filesystem already available in the project