Certs shared service re-usability improvements

Is your feature request related to a problem? Please describe.
Currently the certs shared service implemented as part of #1584 is only usable for one domain at a time (i.e. generating and renewing certificates for nexus-the_id.westeurope.cloudapp.azure.com). It should be improved so it can be used for multiple domains so that it is usable for several concurrent resources for generating and maintaining certs.

Describe the solution you'd like
Currently invoking custom porter actions within the API doesn't allow for parameters, but if this were made possible we could call the "generate" custom action on the certs shared service and pass the fqdn to generate the certificate for, instead of the current implementation where the fqdn needs to be specified when creating the shared service.

The app gateway Terraform as part of the certs bundle could be modified to host multiple domains and additional domains could be passed via a parameter as part of the generate action, which would add the additional pip and associated resources then invoke lets encrypt for that domain.

Tests should also be added to CICD as part of this story to test the certs service is working as expected.

Review the certs shared service and its custom generate action first, then inspect the certs bundle's Application Gateway Terraform. Map how an FQDN parameter would support multiple domains and associated resources, and identify the CI/CD tests needed to verify certificate generation and renewal.