Use OpenID Configuration Discovery Endpoint for SSO/IdP Settings in `reana-server`

[tomondre]

Description:

Currently, the reana-server JWT SSO implementation uses hardcoded or env-provided URLs for token, userinfo, and other IdP endpoints. Instead, we could use the standard OpenID Connect Discovery mechanism to fetch this info dynamically.

Example endpoint:
https://iam-escape.cloud.cnaf.infn.it/.well-known/openid-configuration

Benefits:

• Reduces hardcoded config
• Ensures compatibility with other compliant IdPs
• Automatically pulls updated endpoint URLs (e.g. /token, /devicecode, /userinfo, /jwks_uri, etc.)