Add acm:GetCertificate to policy to provision resources for publish-egress-ip Lambda

[dav3r]

🗣 Description

This PR adds the acm:GetCertificate permission to the policy used to provision resources for the publish-egress-ip Lambda.

💭 Motivation and context

Previously, this permission was not required in order to successfully apply cisagov/publish-egress-ip-terraform, but now it is.

🧪 Testing

I was able to successfully apply this Terraform. I confirmed that the new permission functioned as intended, because I was then able to successfully apply the Terraform in cisagov/publish-egress-ip-terraform.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

[mcdonnnj]

[jsf9k]