Certificate Manager certificate

[Tensho]

The current module version supports classic (old) SSL certificates via Compute service. There is a modern way to manage certificates via Certificate Manager service. Does it make sense to:

1. Create google_certificate_manager_certificate and google_certificate_manager_certificate_map_entry alternatice to existing google_compute_ssl_certificate and google_compute_managed_ssl_certificate resources?
2. Create an optional google_certificate_manager_certificate_map if its name hasn't been provided as input (for the sake of feature completeness)?

Right now, I create a certificate, map and entry with another Terraform module. Adding a new way of managing SSL certificates sounds like a reasonable fit for this module.

I'm committed to contributing to this change.