Enhanced documentation and property dependency for quota in google_identity_platform_config

[modular-magician]

Fixes hashicorp/terraform-provider-google#16172

Release Note Template for Downstream PRs (will be copied)

identityplatform: marked `quota.0.sign_up_quota_config` subfields conditionally required in `google_identity_platform_config` to move errors from apply time up to plan time, and clarified the rule in documentation

Manual testing:
After the change the error message is more clear on the expectations

Sample config:

resource "google_project_service" "identitytoolkit" {
  provider = google-beta  
  project = "my-project"
  service = "identitytoolkit.googleapis.com"
}

resource "google_identity_platform_config" "default" {
  provider = google-beta  
  project = "my-project"
  autodelete_anonymous_users = true
  sign_in {
    allow_duplicate_emails = true

    email {
        enabled = false
    }

    phone_number {
      enabled = false
    }

    anonymous {
      enabled = false
    }
  }
  sms_region_config {
    allowlist_only {
      allowed_regions = [
        "US",
        "CA",
      ]
    }
  }
  quota {
    sign_up_quota_config {
      quota = 100
    }
  }
  authorized_domains = [
    "localhost",
    "pavan-test-terraform.firebaseapp.com",
    "pavan-test-terraform.web.app",
  ]

  depends_on = [ google_project_service.identitytoolkit ]
}

Error before changes:

 Error 400: INVALID_CONFIG : SignUp quota must start between now and 365 days from now.

Error after changes:

│ Error: Missing required argument
│ 
│   with google_identity_platform_config.default,
│   on main.tf line 36, in resource "google_identity_platform_config" "default":
│   36:       quota = 100
│ 
│ "quota.0.sign_up_quota_config.0.quota": all of
│ `quota.0.sign_up_quota_config.0.quota,quota.0.sign_up_quota_config.0.quota_duration,quota.0.sign_up_quota_config.0.start_time` must be specified

Derived from GoogleCloudPlatform/magic-modules#12315