`RuleBindingPolicy` usage

# Feature Request

It's someway complex that we need to rely on 2 different services to do the policy engine settings, e.g.:
- binding policy functions to policy scope it's done on the `PolicyEngine`
- binding left operands to policy scope it's done on the `RuleBindingRegistry`

I think we should expose the methods to bind operands to policy scope in the `PolicyEngine` class, to make the setup more straightforward

## Which Areas Would Be Affected?

policy engine

## Why Is the Feature Desired?

usability

## Who will sponsor this feature?

_Please @-mention the committer that will sponsor your feature_.

## Solution Proposal

_If possible, provide a (brief!) solution proposal._