Compile-time validation for csv_exports entities. For every csv_export entity declared on the resource:

1. The configured read_action exists on the resource and is a :read action.
2. Every column field resolves to an attribute, calculation, or aggregate on the resource.
3. Column headers are unique within the entity.
4. If a column's format: opt is a function, it has arity 1. If it is {module, function, extra_args}, the module exports function/(length(extra_args) + 1).

Failures raise Spark.Error.DslError pointing at the offending entity.