Refactor network module to accept free-form config maps

Replace hypervisor_parent/witness_parent strings with hypervisor_config
and witness_config map(string) variables, matching the pattern already
used by the storage module. incus_networks in the root module is updated
from map(number) to map(object) carrying type and the two config maps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

incus/variables.tf

@@ -13,7 +13,11 @@ variable "incus_token" {
 }
 
 variable "incus_networks" {
-  type = map(number)
+  type = map(object({
+    type              = string
+    hypervisor_config = map(string)
+    witness_config    = map(string)
+  }))
 }
 
 variable "incus_storage_pools" {