feat: Add possibility to override base Issuer

[vdbulcke]

Description

Creating a http listener on 0.0.0.0 address generates invalid Urls on Discovery Endpoint:

ln, err := net.Listen("tcp", ":8080")

generates this as authorization_endpoint

"authorization_endpoint": "http://[::]:8080/oidc/authorize",

Proposed Fix

Add a IssuerBaseUrl string that can be used to specify the base url to reach the mockoidc server.

Then if the IssuerBaseUrl != "" use it to generate the Urls, otherwise default m.Addr() for backward compatibility