Relax recommendation on using literals as rightoperands

[arnoweiss]

Currently, the best-practices are more restrictive than the spec itself [1,2]. The statement was motivated by what's out there today, but the schemas in general allow to use resources as rightOperands as well.

[1]
https://github.com/eclipse-dataspace-protocol-base/dsp_best_practices/blob/45d6381edf7df7984842b4657153beae03333f11/profiling/policy-profiles/README.md?plain=1#L10
[2] https://eclipse-dataspace-protocol-base.github.io/DataspaceProtocol/HEAD/message/schema/contract-schema.json#/definitions/Offer