fix: virtual repository multiple upstream_policies

[romancin]

With current module, defining multiple upstream_policies in the virtual_repository_config variable doesn't work, only the last one is applied.

This PR changes this variable type so upstream_configs is a list. As an example of a working calling to this module using the changes included in this PR:

module "my_repo_3" {
  ...
  mode          = "VIRTUAL_REPOSITORY"
  virtual_repository_config = {
    upstream_policies = [
      {
        id         = module.my_repo_1.artifact_name
        repository = module.my_repo_1.artifact_id
        priority   = 20
      },
      {
        id         = module.my_repo_2.artifact_name
        repository = module.my_repo_2.artifact_id
        priority   = 10
      }
    ]
  }
}

This PR fixes https://github.com/GoogleCloudPlatform/terraform-google-artifact-registry/issues/11

[g-awmalik]

/gcbrun

[g-awmalik]

@romancin - thanks for the PR. Can you please rebase this with master?

[romancin]

@romancin - thanks for the PR. Can you please rebase this with master?

Of course! Done!

[prabhu34]

/gcbrun

[prabhu34]

@romancin - thanks for the PR. Can you please rebase this with master?

Of course! Done!

@romancin I have pushed another change to main. Can you do rebase again?

[romancin]

@romancin - thanks for the PR. Can you please rebase this with master?

Of course! Done!

@romancin I have pushed another change to main. Can you do rebase again?

Of course! Done!

[prabhu34]

/gcbrun