feat: Enhance node group and node addition commands with required flags and validation

- Updated `nodegroup add` command to enforce required flags: `--machine-type`, `--cpu`, `--memory-mb`, `--subnet-id`, `--boot-disk-type`, and `--boot-disk-size-gb`.
- Added validation to ensure specified values for `--cpu`, `--memory-mb`, and `--boot-disk-size-gb` are within acceptable ranges.
- Introduced tests to verify omission of unspecified optional fields in node group addition requests.
- Enhanced error messages for invalid configurations in node addition commands.
- Updated command documentation to reflect new required flags and validation rules.