acme_account: add support for External Account Binding

[felixfontein]

SUMMARY

Fixes #89.

ISSUE TYPE

• Feature Pull Request

COMPONENT NAME

acme_account

[felixfontein]

ready_for_review

[felixfontein]

CC @resmo

[felixfontein]

Hmmm, I can't get this to work with ZeroSSL's ACME server. I always get

{
  "detail": "The request must include a value for the \"externalAccountBinding\" field",
  "status": 400,
  "type": "urn:ietf:params:acme:error:externalAccountRequired"
}

despite externalAccountBinding being included. I guess their error message is very misleading and they want to say that something else is wrong...

[felixfontein]

Ok, I found the "problem": I didn't specify an email during account registration. Once I added an email address, it worked...

[resmo]

LGTM

[felixfontein]

@resmo I added a bit more code to make sure the module is idempotent with ZeroSSL.

[felixfontein]

@resmo thanks a lot for reviewing this one as well!