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/network.auto.tfvars

@@ -1,3 +1,11 @@
 incus_networks = {
-  "main" = 0
+  "main" = {
+    type = "physical"
+    hypervisor_config = {
+      "parent" = "br0"
+    }
+    witness_config = {
+      "parent" = "dummy0"
+    }
+  }
 }