Add DNS validation records to Endpoint Resources

[robertfairhead]

Closes #37 Closes #38

This adds three new fields to the endpoint point resource:

• external_hostname - The public hostname of the load balancer serving this endpoint
• dns_validation_record - The CNAME record that needs to be created for Managed HTTPS to validate
• dns_validation_value - The domain name to which the CNAME record should point for Managed HTTPS to validate

The DNS validation records only appear if Managed HTTPS is being used.

The intended use of the DNS validation records is to feed into a DNS record being managed through Terraform (e.g., Route53) to automate the validation step. There is not a way to trigger an immediate validation of the record in Terraform, because that imperative step does not match Terraform's declarative nature. However, Aptible Deploy will periodically check for the records every few minutes so the validation will eventually occur "automatically" without requiring manual intervention to trigger the validation.

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This relies on the changes in https://github.com/aptible/go-deploy/pull/32 and should not be merged up that PR is released and the module updated here to the released version.

[robertfairhead]

/cc @UserNotFound

[almathew]

Changes look good to me. I will approve once the repository URL has been updated

[robertfairhead]

@almathew It's been updated to the SEMVER tagged version of go-deploy