Support --subnet in network create (#737)

> [!CAUTION]
> Just noticed you can actually create a network that conflicts with an
existing network not managed by `network create`. Unless that is fine do
not merge!

Currently containerization only supports IPv4.

Example:
<img width="2012" height="578" alt="screenshot"
src="https://github.com/user-attachments/assets/38fef1ef-3ffd-4d77-b9e2-8c43f7e4851d"
/>



Docker ref: https://docs.docker.com/reference/cli/docker/network/create/
Podman ref:
https://docs.podman.io/en/v5.0.3/markdown/podman-network-create.1.html#subnet-subnet

Fixes apple/container#458

## Type of Change
- [ ] Bug fix
- [x] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
[Why is this change needed?]

Scripted creation of deterministic network environments for containers

## Testing
- [x] Tested locally
- [ ] Added/updated tests
- [x] Added/updated docs

Sources/ContainerCommands/Network/NetworkCreate.swift

@@ -30,6 +30,9 @@ extension Application {
         @Option(name: .customLong("label"), help: "Set metadata for a network")
         var labels: [String] = []
 
+        @Option(name: .customLong("subnet"), help: "Set subnet for a network")
+        var subnet: String? = nil
+
         @OptionGroup
         var global: Flags.Global
 
@@ -40,7 +43,7 @@ extension Application {
 
         public func run() async throws {
             let parsedLabels = Utility.parseKeyValuePairs(labels)
-            let config = try NetworkConfiguration(id: self.name, mode: .nat, labels: parsedLabels)
+            let config = try NetworkConfiguration(id: self.name, mode: .nat, subnet: subnet, labels: parsedLabels)
             let state = try await ClientNetwork.create(configuration: config)
             print(state.id)
         }

docs/command-reference.md

@@ -527,6 +527,7 @@ container network create NAME [OPTIONS]
 **Options**
 
 *   `--label <key=value>`: set metadata labels on the network
+*   `--subnet <value>`: set subnet on the network
 *   **Global**: `--version`, `-h`/`--help`
 
 ### `container network delete (rm)`