View resulting XML metadata from configuration

[markfullmer]

Background

As raised in comments in #87, it would be useful to view the XML metadata that will be sent to the iDP. This is a comment utility: simpleSAMLphp provides it in its web console (Federation tab -> Show metadata), and the Drupal module implementation provides a dedicated route at /saml/metadata (https://git.drupalcode.org/project/samlauth/-/blob/8.x-3.x/README.md?ref_type=heads#service-provider).

Proposal

Add XML output of the resulting metadata for onelogin/php-saml (the "internal" connection), available at /saml/metadata.