rbac: Implement condition policy validator.

[AndrienkoAleksandr]

Provide a clear and concise description of the feature request

We are going to support condition policy decision feature: https://github.com/janus-idp/backstage-showcase/issues/429 . We need to implement validator for condition objects.

Describe the problem that this feature resolves

That's nice to have opportunity validate condition objects for Backstage permission framework.

Additional context or screenshots for the feature request

Acceptance Criteria