Make compute subnetwork purpose computed

[emilymye]

Purpose should be computed for subnetwork (added in TPGB in hashicorp/terraform-provider-google-beta#1051 it's causing some tests that have private subnetworks to fail from a diff after apply (it's set by API but not in config)

Release Note for Downstream PRs (will be copied)

[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 4f1d8e19673af540bada00552cd00e473d244da9.

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#1124
depends: hashicorp/terraform-provider-google#4427

[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, 8592ec5.

Pull request statuses

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