make transcript string dynamic in ffi_interface

[dragan2234]

It would be good to have a transcript string dynamic (passed as a parameter to create_proof() and verify_proof() methods in ffi_interface. Since test vectors are using different string: https://github.com/jsign/verkle-test-vectors/blob/main/crypto/clients/go-ipa/crypto_test.go#L322

Example in this lib:

rust-verkle/ffi_interface/src/lib.rs

Line 350 in 629d63e

let mut transcript = Transcript::new(b"verkle");