BYO ACM resource

[jamie-lynch]

The GuHttpsClassicLoadBalancer provides a listener prop which accepts an object matching the LoadBalancerListener type. It also sets a number of default values and, unless a value is provided, creates a parameter for the certificate Id. As per @akash1810's comment (below), we could look to replace this parameter with an actual ACM resource instead.

ACM certificates can be created via CloudFormation. Historically, it's been painful as it's a blocking resource - the stack will remain in "updating" until the certificate has been validated.

Validation takes two forms:

If email validated, an email will be sent with a link to be clicked. This email goes to a list of domain admins (around 5 people)
If DNS validated, a CNAME needs created in DNS. Again, admins for the domain service is around 5 people.
However, since the creation of https://github.com/guardian/dns-validation-lambda, it's become more viable to create ACM certs via CFN if domain validated.

(almost at the end of this story!)

Rather than this construct bringing a param for the user to fill in at runtime, should/could it bring an ACM resource with it? Providing the benefit of:

Less involvement for user (aka reduce human error)
Applies best practice of domain validated certificates (best practice because they auto-renew vs. email validated which needs manual renewal once a year)

See original comment at #204 (comment)