Additional `contract.agreement` policy scope

# Feature Request

Currently the contract agreement always happens automatically, the only way obtain "manual approved" flow would be through the implementation of `PendingGuard`s in the negotiation state machine.
This looks not the best option as the policy engine is made for exactly this kind of things.

The proposal is to add a new `contract.agreement` scope with a relative engine evaluation right before the automatic agreement. If this policy doesn't evaluate, the negotiation will be put in a `pending` state, waiting for manual approval (or rejection).

A written proposal in the form of a DR will come up.

## Which Areas Would Be Affected?

_e.g., DPF, CI, build, transfer, etc._

## Why Is the Feature Desired?

_Are there any requirements?_

## Who will sponsor this feature?

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

## Solution Proposal

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