Bump dependency versions, and use Go 1.18. Also add a code scanning s…

…ecurity workflow for development.

Merge pull request #2 from andreas-weber/dev-implement-domain-check-m…

…ethod

Implements method to check for valid dns records

Add Create/Read/Update/Delete methods for domain SMTP credentials

Fix CreateDomain `POST` request parameters

- Oops, domain creation had been failing for a bit since I added
  `whitelabel` and `notification_email` as parameters, but didn't
  re-include `domain`.

Support domain updates too (including whitelabel & notification email)

Add the ability to update email forwards

Domains don't have IDs, but the default alias does

- Return that instead. Otherwise, `Id` is always 0 which is bad for
  Terraform.

Whitelabel domain is only on the Enterprise plan, remove it

- Until someone comes asking for it. ;-)

README: The `ListDomains` functionality is gone

The `Domain` API response isn't an array

- Stupid mistake, I'm tired.