Add purpose and role to subnetwork for L7 load balancing

[tysen]

First of many PRs necessary to support layer 7 HTTP(S) load balancing.

Release Note for Downstream PRs (will be copied)

`google_compute_subnetwork` now includes the `purpose` and `role` fields.

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
This PR seems not to have generated downstream PRs before, as of f9562b6ca897e281de6a128d1461ea186f9d4173.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1051
depends: hashicorp/terraform-provider-google#4261

[slevenick]

extra lines

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 692bdfb.

Pull request statuses

terraform-provider-google-beta already has an open PR.
No diff detected in terraform-google-conversion.
terraform-provider-google already has an open PR.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I didn't open any new pull requests because of this PR.