Add policies validation in PolicyEngine

[wolf4ood]

Feature Request

Introduce a validate method in the PolicyEngine for policies validation.

The validate will take a Policy in input and it will check whenever the policy might contains some
not handled properly by the PolicyEngine within a configuration.

Those checks can be for example:

• an action of a rule is not bound in the RuleBindingRegistry to any scope.
• an atomic constraint left operand is not bound to any functions