Add support for DNS setting verification

[Szasza]

As per the Mailgun API there is a programmatic way to trigger DNS setting verification which would be great to be available via the provider.

Purpose-wise there are three groups of records in Mailgun: sending, receiving, and tracking. It would be ideal to support partial verification as some features may be used, some may not. This however is not supported by the Mailgun API so it would need to be handled in the provider.

Affected Resource(s)

Please list the resources as a list, for example:

• mailgun_domain_dns_verification (new resource)

Example Terraform Configuration

resource "mailgun_domain_dns_verification" "example_com" {
  name = "example.com"

  sending   = true
  receiving = false
  tracking  = true
}

References

Mailgun API documentation: https://documentation.mailgun.com/docs/mailgun/api-reference/openapi-final/tag/Domains/#tag/Domains/operation/httpapi.(*T).Verify-fm-10