Default values for bind address, bind port and redirect URI

[jacoscaz]

This PR takes care of setting reasonable defaults when config options are missing:

• the callback server will bind to an OS-selected port if none is specified via config
• the callback server will bind to address 0.0.0.0 if none is specified via config
• the client will generate the OIDC redirect URI based on the port that the HTTP server has bound and the interfaces reported by the OS if none is specified via config