Prefer X509_USER_{CERT,KEY} over PROXY when discovering X.509 credentials
As a result of the discussion in https://github.com/duncanmmacleod/igwn-auth-utils/pull/5#issuecomment-984663225, this PR modifies the igwn_auth_utils.x509.find_credentials()
function to first look for X509_USER_{CERT,KEY}
and then for X509_USER_PROXY
if required.
cc @lpsinger