Enhance CLI command config

[YuanTingHsieh]

If users type "nvflare config" without entering any information, the old code will just raise an Exception and users see only:

"raise ValueError(f"job_templates_dir='{args.job_templates_dir}', it is not a directory")"

Add a new behavior so if specified nothing, users can check where the file is, and what is the current content of that hidden file.
If any directory is specified, the behavior is the same as before.

Description

• Add print_hidden_config
• Reuse methods from nvflare/utils/cli_utils.py

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[YuanTingHsieh]

/build