Make ResourcePolicy updatable to avoid recreation of it.

[sizzle0121]

The lack of PATCH method results in bugs such as orphaned snapshots when users use Terraform to edit snapshot schedules for disks, since Terraform will plan to delete and recreate a new one, which differs from what we have in our APIs. Therefore, make ResourcePolicy updatable at resource level to fix that.

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Release Note Template for Downstream PRs (will be copied)

resource_policy: make the resource updatable.