compute: support maxPortsPerVm field related to Cloud NAT's enableDynamicPortAllocation

[modular-magician]

Support for the Dynamic Port Allocation feature (tracked in
terraform-google-modules/terraform-google-cloud-nat#64 and
hashicorp/terraform-provider-google#11052) was initially implemented
in #6022, but it lacked support for the maxPortsPerVm field. This
field is crucial to allow the full configuration to work.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: add maxPortsPerVm field to `google_compute_router_nat` resource

Derived from GoogleCloudPlatform/magic-modules#6155