[CDN/CustomDomain] allow to define customHTTPS parameters during creation

Currently a separate API endpoint has to be called to enable CustomHTTPS with custom domains.

This is not ideal as this prevents IaC tooling such as ARM templates, Pulumi and Terraform from using this feature.

I suggest to extend the request body of put to accommodate these properties:

before:

{
  "properties": {
    "hostName": "www.someDomain.net"
  }
}

after:

{
  "properties": {
    "hostName": "www.someDomain.net",
    "customHttpsProvisioning": {
       "certificateSource": "cdn"
       ...
     }
  }
}

Start with the linked CustomHTTPS enable-custom-https REST endpoint and the current custom-domain request schema in the Azure API specifications. Trace how the request body is defined and generated, then verify that customHttpsProvisioning is accepted during creation and that the resulting specification remains valid.