Add nested relational field filtering

[kettasoft]

Summary

Add support for filtering relational fields using nested request arrays in the Ruleset and Expression engines.

Changes

• Add a relation-aware nested field parser.
• Support nested and dot-notated relational fields.
• Preserve operator expressions, structured conditions, list values, and empty arrays.
• Support direct, field-specific, wildcard, deep, and mixed relation definitions.
• Improve wildcard handling for allowed fields.
• Add unit and integration coverage.
• Update the README and engine documentation.