Unbreak oauth2-proxy for keycloak provider after 2c668a (#1502)

* Unbreak oauth2-proxy for keycloak provider after 2c668a

With 2c668a, oauth2-proxy fails a request if the token validation fails.
Token validation always fails with the keycloak provider, due to the
valudation request passing the token via the URL, and keycloak not
parsing the url for tokens.

This is fixed by forcing the validation request to pass the token via a
header.

This code taken from the DigitalOcean provider, which presumably forcing
the token to be passed via header for the same reason.

Test plan: I was unable to build a docker image to test the fix, but I
believe it is relatively simple, and it passes the "looks good to me"
test plan.

* Add changelog entry for unbreak keycloak

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>