bug: required policy arguments are bypassed when used with input bindings

[jiparis]

In the context of for example a policy group:

apiVersion: workflowcontract.chainloop.dev/v1
kind: PolicyGroup
metadata:
  name: my-group
spec:
  inputs:
    - name: foo
policies:
  attestation:
    - ref: my-policy
      with: 
        policyfoo: "{{ inputs.foo }}"

In this example, if policyfoo argument is required, validation won't take into account that it might have input bindings on it. This will cause a false positive if the group is instantiated without that parameter

# my contract
policyGroups:
  - ref: my-group

The above example contract won't fail, even if policyfoo is mandatory and it doesn't have any value