Add support for ZeroSSL account registration

[fln]

This commit extends lego library and cli tool to support issuing certificates from ZeroSSL without having to manually create an account.

Without this commit ZeroSSL can be used but users need to manually create ZeroSSL account and start lego in EAB (External Account Binding) mode.

From the lego cli tool perspective this commit:

Detects if lego ir running with ZeroSSL ACME directory --server https://acme.zerossl.com/v2/DV90 and uses ZeroSSL API to issue keys for EAB. There is no need to provide --eab, --kid, --hmac values anymore.

From the library perspective this commit:

Creates new method RegisterWithZeroSSL() in the registration package which takes care of creating ZeroSSL account with a given email. Internally it re-uses RegisterWithExternalAccountBinding() method after KID and HMAC are retrieved from ZeroSSL registration endpoint.

[ashkitten]

i would also like this feature. i've actually rebased this patch and am using it already for myself.