Add --max-concurrent-downloads flag for parallel layer downloads (#716)

Adds `--max-concurrent-downloads` flag to `container image pull` for
configurable concurrent layer downloads.

Fixes #715
Depends on apple/containerization#311

**Usage**:
```bash
container image pull nginx:latest --max-concurrent-downloads 6
```

**Changes**:
- Add CLI flag (default: 3)
- Thread parameter through XPC stack
- Update to use forked containerization with configurable concurrency

**Performance**: ~1.2-1.3x faster pulls for multi-layer images with
higher concurrency

**Tests**: Included standalone tests verify concurrency behavior and
parameter flow

---------

Co-authored-by: Claude <noreply@anthropic.com>

Sources/ContainerCommands/Container/ContainerCreate.swift

@@ -40,6 +40,9 @@ extension Application {
         @OptionGroup(title: "Registry options")
         var registryFlags: Flags.Registry
 
+        @OptionGroup(title: "Image fetch options")
+        var imageFetchFlags: Flags.ImageFetch
+
         @OptionGroup
         var global: Flags.Global
 
@@ -73,6 +76,7 @@ extension Application {
                 management: managementFlags,
                 resource: resourceFlags,
                 registry: registryFlags,
+                imageFetch: imageFetchFlags,
                 progressUpdate: progress.handler
             )