Fix for non automatic domain authentication dns records

[andre-vspry]

If the automatic_security attribute of sendgrid_domain_authentication is not set then the dns records are not cnames and don't match the existing response format.

e.g.

resource "sendgrid_domain_authentication" "default" {
    domain = trimsuffix(google_dns_record_set.a.name, ".")
    # ips = [ "10.10.10.10" ]
    # is_default = true
    # automatic_security = true
}

Added tests and extended to handle txt and mx dns records.

Also added debug flag to main to enable debugging with delve.