Support receiving the ssh key base64 encoded
Depending on the authentication server being used and its backend
it can be that the ssh keys provided are in plain text or base64
encoded, so we want to support both.
The support for this using OIDC was added in
5d86053abc278fb5448b508cee92bcd1beb3e3c8
This commit adds the same logic for OpenID.
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>