Use in-toto/attestation Go bindings for Statement and SLSA provenance predicate layers

[marcelamelara]

To ensure better consistency between different in-toto implementations of the ITE-6 spec, in-toto/attestation generates language bindings for the latest Statement and Predicate layers based on protobuf, including Go bindings. in-toto-golang currently implements the Statement and SLSA provenance predicate layers separately, though per #221 (comment), the preference would ultimately be to transition to using the canonical in-toto/attestation bindings for Statement and Predicate layers.