CloudNGFW module should support subnet_mapping according to the endpoint mode

[alperenkose]

Is your feature request related to a problem?

cloudngfw resource subnet_mapping is currently hardcoded to availability_zone as of now which is only for "CustomerManaged" endpoint:

terraform-aws-swfw-modules/modules/cloudngfw/main.tf

Line 20 in e400b5e

availability_zone = subnet_mapping.key

Describe the solution you'd like

We should support dynamic subnet_mapping depending on the type of endpoint_mode, as I understand "ServiceManaged" mode requires subnet_id whereas "CustomerManaged" mode requires availability_zone or availability_zone_id

Describe alternatives you've considered.

No response

Additional context

No response