[auth_oidc] URL endpoints should be autodetected via OpenId Connect Discovery

[OdyX]

Is your feature request related to a problem?
No. It's only that the manual setting of the 4 (or 5) URLs is cumbersome, error-prone and can be circumvented.

Describe the solution you'd like

The configuration should ask for the "Issuer URL", and rely on the "OpenID Connect Discovery 1.0" protocol to find all other URLs (via .well-known/openid-configuration).

It should be possible to support both the "one URL" and "customize every URL" modes.