provide a way to get other endpoint URLs

[ymajoros]

There seems to be currently no way of getting endpoint URLs from the discovery document for some entries, e.g.: endSessionEndpoint etc.

As not all endpoints are specified and we can expect new ones to arise, maybe it would be a good option to be able to specify a free parameter for endpoints besides the existing enum.

[evert]

I assume these are openid-specific endpoints? Perhaps we can just extend the current enum to cover these as well

[ymajoros]

Extending the enum would help, but there can still be free entries, either from future or related specifications (e.g. side-channel login) or proprietary. Keycloak has a lot more, for example.