Support request claims, such as a sooner-than-default exp

[coolaj86]

For example:
When manually debugging auth flows it's nice to be able to expire the session early.

If I'm reading the docs correctly, that will look like this:

{
    "claims": {
        "access_token": {
            "exp": {
                "essential": true,
                "value": "1m"
            }
        }
    }
}

See https://openid.net/specs/openid-connect-core-1_0.html#ClaimsParameter