Add command to switch "autobuild" on/off

Hi,

most of the time I run maven build outside of vscode in the terminal. This interferes with the extension's way of detecting changes. It seems that the extension also starts building, which leads to alot of ClassNotFoundExceptions in the maven build (probably because vscode deleted the already build classes while the maven build was running).

To circumvent this I used to temporarily disable the auto build feature back when I was using Eclipse.
I know this is also possible via the settings menu.

Environment

• Operating System: macOS 11.5.1 (20G80)
• JDK version: openjdk version "15" 2020-09-15
• Visual Studio Code version: 1.60.0-insider
• Java extension version: v0.18.2

Steps To Reproduce

1. Start a long running maven build (something simple will not work)
2. See ClassNotFoundExceptions in the maven build

Expected Result

• Add the `Autobuild: Enabled/Disabled" settings to the command pallet
• Add the `Autobuild: Enabled/Disabled" to the status bar, at least to show the status, better even to allow to toggle the setting.

This would allow to easily switch between those two modes.

该 issue 指向现有的 autobuild 设置及其在 settings-menu 中的实现；首先定位该设置，以及扩展的命令和状态栏注册位置。验证长时间运行的 Maven 复现，然后确认可以从 Command Palette 和状态栏切换该设置，并且其启用或禁用状态是可见的。