Make google_compute_security_policy's rule_visibility optional + computed and remove default value

[modular-magician]

Supersedes GoogleCloudPlatform/magic-modules#8060
Might fix #12743 ?

I find that issue above confusing as it includes 2 issues:

1. Being unable to create a google_compute_security_policy resource when this is in the config:

adaptive_protection_config {
      layer_7_ddos_defense_config {
          enable = false
      }
}

2. Issues/expected behaviour when the adaptive_protection_config block is removed:

- adaptive_protection_config {
-      layer_7_ddos_defense_config {
-          enable = false
-      }
-}

This PR addresses issue 1.
I explored issue 2 but decided that it'd be best to address that separately to 1.

Release Note Template for Downstream PRs (will be copied)

compute: fixed issue where users could not create `google_compute_security_policy` resources with `layer_7_ddos_defense_config` explicitly disabled

Derived from GoogleCloudPlatform/magic-modules#10823

[github-actions]

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