Add optional metadata validation for module manifests

[Tanvip2602]

Description

Adds optional metadata validation for OpenCRAVAT module info.yaml manifests.

1.Changes

• Added validate_manifest.py for manifest validation.
• Added validation for optional fields:
  • homepage
  • contact
  • license
• Missing optional fields generate warnings instead of errors.
• Added pytest unit tests for complete and incomplete manifests.
• Added usage instructions to tests/README.md.

2. Testing

• Tested complete manifests.
• Tested manifests with missing optional fields.
• Tested missing required fields.
• Tested invalid YAML.
• All 4 unit tests passed.

Closes #370

[kmoad]

Thanks for the PR! We'll look it over and get back to you soon.

[kmoad]

This looks good and fulfills requirements.

Could you move it under the cravat directory, instead or project root.

After that, we think it's good to merge.