example(policy): New json-field-validator policy example

[danlishka]

Summary

Adds a comprehensive JSON field validation policy example with shared testing utilities and improved policy structure.

Changes

New Policy Example

• json-field-validator/: Complete policy example for validating JSON fields
• Generic field validation with dot notation support (application.name, security.enabled)
• Configurable validation types (exact values, regex patterns)
• Works with EVIDENCE material type for JSON files

Enhanced Testing Framework

• Shared _testutils.sh: Configurable test utilities supporting multiple material types
• Proper exit codes (0 for pass, 1 for fail)
• Skipped/ignored policy detection
• Custom material type support (--kind parameter)

Additional Improvements

• sbom-freshness/: Restructured SBOM policy following new template

Policy Features

• Field Validation: required_field=application.name expected_value=web-service
• Pattern Matching: required_field=version field_pattern="^[0-9]+\.[0-9]+\.[0-9]+$"
• Nested Fields: Supports dot notation for nested JSON objects
• Generic Functions: Reusable field_value() and field_exists() helpers

Testing

  # All policies tested and working
  ./json-field-validator/test.sh 
  ./sbom-freshness/test.sh 

[migmartri]

DCO check is failed, please make sure you signoff your commits, thanks!

[migmartri]

Thanks,

Do you want the json policy to be present just in an example or moved upstream to chainloop platform?