Expose Command Structs for Plugins (#603)

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

## Motivation and Context
Plugins technically exist, but to add shortcuts or to do existing things
with functions in `container` requires calling a compiled binary. This
pull request aims to remove that hurdle and instability by exposing
commands as a new `ContainerCommands ` target.

Simply import `ContainerCommands` and you can access almost
any command as if it were a native part of the binary. This makes
plugin development significantly easier.

Closes #609.

Sources/ExecutableCLI/Executable.swift

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025 Apple Inc. and the container project authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import ContainerClient
+import ContainerCommands
+
+@main
+public struct Executable: AsyncParsableCommand {
+    public init() {}
+
+    @Argument(parsing: .captureForPassthrough)
+    var arguments: [String] = []
+
+    public static let configuration = Application.configuration
+
+    public static func main() async throws {
+        try await Application.main()
+    }
+
+    public func run() async throws {
+        var application = try Application.parse(arguments)
+        try application.validate()
+        try application.run()
+    }
+}