fix: [dns] fix hash function for network_url in ManagedZone and Policy resources

[modular-magician]

When someone is trying to attach a private DNS zone to multiple networks in different projects, if the network name is the same across some of those projects, the resource will only attach only one of that networks with the same name, even if they are on different projects.

Example:

• Project project1 with networks network1 and network2 to be attached
• Project project2 with networks network2 and network3 to be attached

The Terraform resource will only try to attach network1 from project1, network2 and network3 from project3 thus ignoring network2 from project1

Updating the way how the resource is handling the hash will fix this issue.

resource "google_dns_managed_zone" "x" {
  dns_name      = "dummy.com."
  force_destroy = true
  name          = "dummy.com"
  project       = "dummy-project"
  visibility    = "private"

  cloud_logging_config {
    enable_logging = false
  }

  private_visibility_config {
    networks {
      network_url = "https://www.googleapis.com/compute/v1/projects/project1/global/networks/network1"
    }
    networks {
      network_url = "https://www.googleapis.com/compute/v1/projects/project1/global/networks/network2"
    }
    networks {
      network_url = "https://www.googleapis.com/compute/v1/projects/project2/global/networks/network2"
    }
    networks {
      network_url = "https://www.googleapis.com/compute/v1/projects/project2/global/networks/network3"
    }
  }
}

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

dns: fix hash function for network_url in ManagedZone and Policy resources

Derived from GoogleCloudPlatform/magic-modules#8716

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.