Enable override on custom landing zone to Brownfield migration

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### Description

In order to support brownfield solution migration, there is a need to support archetype_config_oveerides for custom landing zone.

This will allow customer to create a Staging custom landing zone and deploy the same policy assignment as root/landing zone where it is enforced, but use the override with 'DoNotEnforce' in the Staging management group.

This approach would simplify and reduce potential errors by avoiding creating / maintaining similar policy assignment

#### Is your feature request related to a problem?

The variable "archetype_config_overrides" does not work for management groups specified by the 'custom_landing_zones' input variable as documented [here](https://github.com/Azure/terraform-azurerm-caf-enterprise-scale/blob/76a1c6dd1df2947d10ded7516cdbe1e6d0ed5294/variables.tf#L427).

#### Describe the solution you'd like

Enable "archetype_config_overrides" for 'custom_landing_zones'

A potential simple workaround would be to add a Staging management group at the same level as the Landing Zone in the core Mangement Group IDs.

#### Additional context

Start with variables.tf and the custom_landing_zones input referenced in the issue, then trace how archetype_config_overrides is applied to standard landing zones. Compare that path with custom landing zone management groups. Done means custom landing zones can use archetype_config_overrides, including a DoNotEnforce override for a staging management group, without duplicating policy assignments.

Written by the indexing model from the issue text.