Add new authorization strategy that works with OIDC (e.g. Keycloak)

[amadio]

The use case for this is to use tokens from CERN's SSO (Keycloak) with SWAN, by mapping users based on the username_claim (cern_upn, for example, in the CERN SSO tokens). One could imagine also configuring access based on other online accounts such as from Google, GitHub, etc.

The idea which we discussed offline is to add a new authorization strategy in scitokens-cpp that would allow to map tokens to users directly, without a mapfile (which seems to be currently required for the existing mapping authorization strategy). The new strategy could simply check for openid scope, and map sub or the username_claim to the user name. If a strategy to support OAuth2 is possible, that would be even better.