Add DeDi backed native support for trusted smart contracts (auto-invoicing) on fabric

[ameetdesh]

• Create beckn smart Policy as first class schema with digest, rateability, DeDi backed signing support.
• Redesign rego backed auto-invoicing as on-fabric smart contract, to eliminate any friction or code injection risk.
• Onix should keep audit trails about url accessibility, checksum etc.
• Get @anandp504 and @ravi-prakash-v 's and @nirmalnr 's help in breaking this down into sub-issues.