Explicitly store LE account ID

[valb3r]

Due to some issues on LE side it is useful to be able to access LE accountID in unencrypted way for diagnostics (to prevent future issues):
Example issue as motivation (it does not affect this lib)

Hello,

Please immediately renew your TLS certificate(s) that were issued from
Let's Encrypt using the TLS-ALPN-01 validation method and the following
ACME registration (account) ID(s):

 111111111  22222222 

We've determined that an error made it possible for TLS-ALPN-01
challenges, completed before today, to not comply with certificate
issuance requirements. We have remediated this problem and will revoke
all unexpired certificates that used this validation method at 16:00 UTC
on 28 January 2022. Please renew your certificates now to ensure an
uninterrupted experience for your site visitors.

We apologize for any inconvenience this may cause. If you need support
in the renewal process, please comment on our forum post. Our staff and
community members are available to help:

ID 111111111 should be accessible for diagnostics