Do not sign the transfer transaction on mobile

[mpoplavkov]

The current implementation of the build transfer transaction method signs the transaction:

fuels-dart/packages/fuels/native/src/features/transaction.rs

Line 115 in 66f0c2a

wallet.add_witnessses(&mut tx_builder);

Without that, the call of tx_builder.build would fail, because it requires signatures to be present.

This makes it insecure to send such signed transaction requests over network without the desire to execute them (eg for simulation)

See the issue on forum