validatron rewrite: rules engine improvements

[banditopazzo]

validatron rewrite: rules engine improvements

This pr is a complete rewrite of validatron crate with all derived changes.

• Fix dsl: relative event fields as values #172
• Collection support (CONTAINS) as mentioned in rule engine dsl syntax changes #71, so closes dsl collection support #112 where there is the previous attempt

Implementation (Optional)

• Generalized validatron with a single trait instead of previously multiple traits (for struct and for enum)
• Splitted the previous all-in-one field validation method in two components
  • a sort of reflection infrastructure
  • the validation process that leverage the previous point; this made possible to have dynamic field rules and better handle collection types
• Moved the dsl from validatron to the rules engine, as it's specifically designed to work with our rules

I have

• run cargo fmt;
• run cargo clippy;
• run cargo testand all tests pass;
• linked to the originating issue (if applicable).