Add support for RFC 9207 (?)

[mlazowik]

I'm not 100% sure whether this is something that pyoidc should handle. I'm adding the issue already in case for someone else it's obvious. If there are no responses I'll research more and update when I find the time.

In particular Keycloak v23 implemented that RFC and now our app is receiving two iss keys in the url params in auth response, which ends up throwing TooManyValues from

pyoidc/src/oic/oauth2/message.py

Line 296 in 0e30cd2

raise TooManyValues("{}".format(key))

For now we'll be using the Exclude Issuer From Authentication Response compatibility option in Keycloak.

[tpazderka]

This should probably be implemented as an extension message...