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/modules/network/variables.tf

@@ -15,12 +15,12 @@ variable "witnesses" {
   type = set(string)
 }
 
-variable "hypervisor_parent" {
-  type    = string
-  default = "br0"
+variable "hypervisor_config" {
+  type    = map(string)
+  default = {}
 }
 
-variable "witness_parent" {
-  type    = string
-  default = "dummy0"
+variable "witness_config" {
+  type    = map(string)
+  default = {}
 }