Add option to disable SPF TXT record

[fmasuhr]

Adding configure_spf variable to allow disabling the SPF TXT record. This way the TXT record can be managed outside of the module.

This change will re-create the TXT record as it now is using count

[asayed94]

Thanks @fmasuhr I think this could solve the problem for forcing new SPF records

[awendt]

This means we'll end up with 2 types of consumer modules, and the responsibility for the SPF record will always be shared.

We can force the responsibility to always be with the module if we make an array of domains configurable:

# variables.tf
variable "spf_additional_domain_names" {
  type    = list(string)
  default = []

  description = <<EOS
Domain names to be included in the SPF record.
EOS
}

When I specify spf_additional_domain_names = ["emarsys.us"], the module could end up creating the SPF record: v=spf1 include:amazonses.com include:emarsys.us ~all

@fmasuhr What do you think?

[jansiwy]

@awendt By design, an SPF record is not specific a specific ESP. It only work for simples cases with one ESP per domain. Thus, a module for a specific ESP should not pretend, that it's always responsible for the SPF record (this would obviously clash if modules for different ESPs would both apply that pattern).