feat(AztecNr): create byte utils for L1<>L2 messages

[rahul-kothari]

Similar to abi.encodeWithSelector - makes L1<>L2 messages easier

// Compute sha256 hash to a field for a message with a 4 bytes of function selector and a number of arguments
// Useful for content hash used in L1<>L2 messages
fn compute_sha256_hash_for_function_signature<N>(selector: Field, args: [Field; N]) -> Field {
  let len = 4 + (N*32);
  let mut bytes_to_hash: [u8; len] = [0; len];

  // copy first 4 bytes of selector:
  let selector_in_bytes = selector.to_be_bytes(4);
  for i in 0..4 {
    bytes_to_hash[i] = selector_in_bytes[i];
  }

  // copy the args:
  for i in 0..N {
    let offset = 4 + i*32;
    let arg_in_bytes = args[i].to_be_bytes(32);
    for j in 0..32 {
      bytes_to_hash[offset + j] = arg_in_bytes[j];
    }
  }
  sha256_to_field(bytes_to_hash)
}