Change behavior of the default arguments in the plugin config (#1063)

## Type of Change
- [x] New feature  
- [x] Breaking change

## Motivation and Context
This PR changes the behavior of the `defaultArguments` field in the
service plugin config. Previously, `defaultArguments` was functioning as
a way to indicate how to start a plugin in the event that the plugin is
loaded at boot (such as for the `container-core-images plugin`).
However, we now follow a convention where all plugins have a "start"
command that is used when launching the plugin, so this
`defaultArguments` field wasn't really providing much. Instead, there
are use cases where we may want to set default values to pass to a
plugin. This PR repurposes the `defaultArguments` field for those use
cases.

As an example use case, there are scenarios where someone may want to
use the AllocationOnlyVmnetNetwork even when running on macOS 26+. This
PR adds the ability to pass in a command line option to the vmnet
network plugin to specify that request. Combined with the
`defaultArguments` plugin config change, a user may choose to set that
field to ["--variant", "allocationOnly"] in the
`container-network-vmnet-config.json` to use AllocationOnlyVmnetNetwork
by default for all networks.

## Testing
- [x] Tested locally

Sources/ContainerPlugin/PluginConfig.swift

@@ -79,7 +79,6 @@ public struct PluginConfig: Sendable, Codable {
         public let services: [Service]
         /// An optional parameter that include any command line arguments
         /// that must be passed to the plugin binary when it is loaded.
-        /// This parameter is used only when `servicesConfig.loadAtBoot` is `true`
         public let defaultArguments: [String]
     }
 

Sources/ContainerPlugin/PluginLoader.swift

@@ -225,7 +225,7 @@ extension PluginLoader {
 
         let plist = LaunchPlist(
             label: id,
-            arguments: [plugin.binaryURL.path] + (args ?? serviceConfig.defaultArguments),
+            arguments: [plugin.binaryURL.path] + (args ?? ["start"]) + serviceConfig.defaultArguments,
             environment: env,
             limitLoadToSessionType: [.Aqua, .Background, .System],
             runAtLoad: serviceConfig.runAtLoad,

Sources/Helpers/NetworkVmnet/NetworkVmnetHelper+Start.swift

@@ -19,10 +19,16 @@ import ContainerNetworkService
 import ContainerNetworkServiceClient
 import ContainerResource
 import ContainerXPC
+import ContainerizationError
 import ContainerizationExtras
 import Foundation
 import Logging
 
+enum Variant: String, ExpressibleByArgument {
+    case reserved
+    case allocationOnly
+}
+
 extension NetworkVmnetHelper {
     struct Start: AsyncParsableCommand {
         static let configuration = CommandConfiguration(
@@ -45,6 +51,14 @@ extension NetworkVmnetHelper {
         @Option(name: .customLong("subnet-v6"), help: "CIDR address for the IPv6 prefix")
         var ipv6Subnet: String?
 
+        @Option(name: .long, help: "Variant of the network helper to use.")
+        var variant: Variant = {
+            guard #available(macOS 26, *) else {
+                return .allocationOnly
+            }
+            return .reserved
+        }()
+
         func run() async throws {
             let commandName = NetworkVmnetHelper._commandName
             let log = setupLogger(id: id, debug: debug)
@@ -63,7 +77,11 @@ extension NetworkVmnetHelper {
                     ipv4Subnet: ipv4Subnet,
                     ipv6Subnet: ipv6Subnet,
                 )
-                let network = try Self.createNetwork(configuration: configuration, log: log)
+                let network = try Self.createNetwork(
+                    configuration: configuration,
+                    variant: self.variant,
+                    log: log
+                )
                 try await network.start()
                 let server = try await NetworkService(network: network, log: log)
                 let xpc = XPCServer(
@@ -86,12 +104,19 @@ extension NetworkVmnetHelper {
             }
         }
 
-        private static func createNetwork(configuration: NetworkConfiguration, log: Logger) throws -> Network {
-            guard #available(macOS 26, *) else {
+        private static func createNetwork(configuration: NetworkConfiguration, variant: Variant, log: Logger) throws -> Network {
+            switch variant {
+            case .allocationOnly:
                 return try AllocationOnlyVmnetNetwork(configuration: configuration, log: log)
+            case .reserved:
+                guard #available(macOS 26, *) else {
+                    throw ContainerizationError(
+                        .invalidArgument,
+                        message: "variant ReservedVmnetNetwork is only available on macOS 26+"
+                    )
+                }
+                return try ReservedVmnetNetwork(configuration: configuration, log: log)
             }
-
-            return try ReservedVmnetNetwork(configuration: configuration, log: log)
         }
     }
 }

config/container-core-images-config.json

@@ -11,6 +11,6 @@
                 "description": "Provide an XPC interface to interact with an image store."
             }
         ],
-        "defaultArguments": ["start"]
+        "defaultArguments": []
     }
 }