[commands] Add DeferredCommand (#5566)

Allows commands to be constructed at runtime without proxying.

wpilibNewCommands/src/main/native/cpp/frc2/command/DeferredCommand.cpp

@@ -0,0 +1,46 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "frc2/command/DeferredCommand.h"
+
+#include <wpi/sendable/SendableBuilder.h>
+
+#include "frc2/command/Commands.h"
+
+using namespace frc2;
+
+DeferredCommand::DeferredCommand(wpi::unique_function<CommandPtr()> supplier,
+                                 Requirements requirements)
+    : m_supplier{std::move(supplier)} {
+  AddRequirements(requirements);
+}
+
+void DeferredCommand::Initialize() {
+  m_command = m_supplier().Unwrap();
+  CommandScheduler::GetInstance().RequireUngrouped(m_command.get());
+  m_command->SetComposed(true);
+  m_command->Initialize();
+}
+
+void DeferredCommand::Execute() {
+  m_command->Execute();
+}
+
+void DeferredCommand::End(bool interrupted) {
+  m_command->End(interrupted);
+  m_command =
+      cmd::Print("[DeferredCommand] Lifecycle function called out-of-order!")
+          .WithName("none")
+          .Unwrap();
+}
+
+bool DeferredCommand::IsFinished() {
+  return m_command->IsFinished();
+}
+
+void DeferredCommand::InitSendable(wpi::SendableBuilder& builder) {
+  Command::InitSendable(builder);
+  builder.AddStringProperty(
+      "deferred", [this] { return m_command->GetName(); }, nullptr);
+}