feat: add `envd shutdown` command to deinitialize envd

[Electronic-Waste]

Describe the feature

envd shutdown: kill envd_buildkitd container when no envd enviroments exist .
envd shutdown -f: kill envd_buildkitd container and all running envd environments.

Why do you need this feature?

There is no command like envd shutdown to deinitialize envd, which allows users closing envd as they want, keeping the development environment tidy and neat.

Additional context

No response

[kemingy]

We have envd v1 which can use moby builder directly (doesn't need to run a buildkitd container). I think that's the trend. However, we don't have a plan to release v1 recently.

[Electronic-Waste]

Okay, that's fine.