The credentials for external account binding can only be entered
interactively when registering an ACME account with pvenode, and only
if the CA requires them or a custom directory was selected. So they
cannot be passed when the account is registered from a script, or
when the directory is passed with --directory and the CA does not
announce that it requires them.
Add the --eab-kid and --eab-hmac-key options, which the API endpoint
already supports, and only ask for the credentials if they were not
passed.
Signed-off-by: Michal Fox <me@dualfroz.com>