sg: cloud - move all cloud ephemeral commands to a sub command `ephemeral`

[burmudar]

go run ./dev/sg cloud --help
NAME:
   sg cloud - Install and work with Sourcegraph Cloud tools

USAGE:
   sg cloud command [command options] [arguments...]

DESCRIPTION:
   Learn more about Sourcegraph Cloud:

   - Product: https://sourcegraph.com/docs/cloud
   - Handbook: https://handbook.sourcegraph.com/departments/cloud/


COMMANDS:
   install         Install or upgrade local `mi2` CLI (for Cloud V2)
   ephemeral, eph  Set of commands that operate on Cloud Ephemeral instances
   help, h         Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help

And when you do sg cloud eph

go run ./dev/sg cloud eph --help
NAME:
   sg cloud ephemeral - Set of commands that operate on Cloud Ephemeral instances

USAGE:
   sg cloud ephemeral command [command options] [arguments...]

DESCRIPTION:
   Commands to create, inspect or upgrade Cloud Ephemeral instances

COMMANDS:
   build          Trigger a cloud ephemeral build which will push images to the cloud ephemeral registry
   delete         Delete an ephemeral instance
   deploy         Deploy a new ephemeral instance from the current branch or specific version
   lease          Extend or reduce the lease expiry time of an ephemeral instance
   list           List ephemeral instances assigned to you, or all instances
   list-versions  List docker images in the cloud ephemeral registry
   status         Get the status of a ephemeral instance
   upgrade        upgrade an ephemeral instance
   help, h        Shows a list of commands or help for one command

Test plan

Tested locally