Update set_finalizers host function to take a packed_finalizer_format

[heifner]

set_finalizers host function now takes a packed_finalizer_format to allow for future changes to packed format of finalizer policy.

         /**
          * Submits a finalizer set change.
          *
          *  // V0 format for packed finalizer_policy
          *  struct finalizer_authority {
          *     std::string              description;
          *     uint64_t                 weight = 0; // weight that this finalizer's vote has for meeting fthreshold
          *     std::vector<uint8_t>     public_key; // Affine little endian non-montgomery g1, cdt/abi_serializer has issues with std::array, size 96
          *  };
          *  struct finalizer_policy {
          *     uint64_t                          threshold = 0;
          *     std::vector<finalizer_authority>  finalizers;
          *  };
          *
          * Valid formats:
          * 0 : serialized finalizer_policy
          *
          * @ingroup privileged
          *
          * @param packed_finalizer_format - format of the finalizer_policy data blob.
          * @param packed_finalizer_policy - a serialized finalizer_policy object.
         */
         void set_finalizers(uint64_t packed_finalizer_format, span<const char> packed_finalizer_policy);

Depends on AntelopeIO/cdt#278 for compiling contracts.

Resolves #97

[ericpassmore]

Note:start
group: IF
category: INTERNALS
summary: Improve set_finalizers host function to take a packed finalizer format.
Note:end