Fix sql_database_instance to include backup_configuration in create request

[modular-magician]

Fixes b/257534640 (TF plan allows google_sql_database_instance.replica to define a backup configuration)

SQL database instances do not allow replicas to enable backups. For example, the following config will fail:

resource "google_sql_database_instance" "main" {
  name             = "main-instance"
  ...
}

resource "google_sql_database_instance" "replica" {
  name             = "replica-instance"

  # Defines instance as a replica
  master_instance_name = "${google_sql_database_instance.main.name}"    
  replica_configuration {
    failover_target = false
  }

  settings {
    ...
    # Enables backups
    backup_configuration {
      enabled = true
      point_in_time_recovery_enabled = true
    }
  }
}

Problem
Previously, config like this would correctly result in an error, but the replica would still be created with the backup_configuration omitted. This caused issues with consecutive terraform apply calls where the first succeeded, but the second failed with no changes to the config.

Root cause
We were using some custom behavior for backup_configuration.binary_log_enabled that removed it from the initial create request, and then added it back to the instance with a subsequent patch request. This behavior had a bug that caused the entire backup_configuration to be removed and not get patched if binary_log_enabled was false for a replica.

Resolution
This PR proposes fixing the issue by leaving the backup_configuration on the create request, so that the server can validate the full configuration before creating any entities. The one exception is binary_log_enabled, which cannot be set to true until after a replica is created. In that case, we make the initial create request with binary_log_enabled = false, and then use the subsequent patch call to set it to true like before.

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Release Note Template for Downstream PRs (will be copied)

sql: fix `googe_sql_database_instance` to include `backup_configuration` in initial create request

Derived from GoogleCloudPlatform/magic-modules#6836

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