feat(variable): Allow updating variables from .env file

[pan93412]

Description (required)

Add a new command: variable env

$ zeabur variable env --help
overwrite variables from a .env file

Usage:
  zeabur variable env [flags]

Flags:
      --env-id string   Environment ID of service
  -f, --file string     Path to the .env file (default ".env")
  -h, --help            help for env
      --id string       Service ID
  -n, --name string     Service name

Global Flags:
      --auto_check_update    automatically check update (default true)
      --auto_refresh_token   automatically refresh token when it's expired, only works when the token is from browser(OAuth2) (default true)
      --debug                Enable debug logging
  -i, --interactive          use interactive mode (default true)

The service variables are overridden with the content of the provided .env file.

Related issues & labels (optional)

• Closes ZEA-3754
• Suggested label: enhancement