Add rules converter

What does this MR do and why?

Adds RulesConverter to convert expression AST nodes to JSON

Example:

'$[[ inputs.env ]] == "prod" && $[[ inputs.region ]] == "us"'

RulesConverter transforms the parsed AST into:

{
  "operator": "AND",
  "children": [
    { "operator": "equals", "field": "env", "value": "prod" },
    { "operator": "equals", "field": "region", "value": "us" }
  ]
}

References

#563376

MR	Status	Description
Input lexeme	!206891 (merged)	Parse $[[ inputs.X ]] syntax in expressions
Rules converter	👈 You are here	Convert expression AST to JSON
Semantic validation	In review !209496	Validate input references and circular dependencies, error handlin
GraphQL API	In review next, needs: input lexme, rules converter	Expose rules with condition trees

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