Add environment:metadata command.

It may be a command too far, but this could be useful for scripts - e.g.
```bash
COMMAND="platform environment:metadata --environment $ENV_ID --project $PROJECT_ID --pipe "

echo "The environment's title is: $($COMMAND title)"
echo "The environment's parent is: $($COMMAND parent)"
echo "The environment is $($COMMAND status)"
echo "The environment's HEAD is: $($COMMAND head_commit)"
```