Feature requests around passing in signatures and messages to predicates

[Dhaiwat10]

Some feature requests that I thought of while trying to build an app that must pass a message and the same message signed by a wallet to a predicate. Repo here: https://github.com/Dhaiwat10/fuel-session-keys-demo

1. The code looks like this right now:

 const messageToSign = Uint8Array.from([
      ...toUtf8Bytes("gFUEL"),
      ...new Uint8Array(27),
    ]);

    // @ts-expect-error signMessage does not accept Uint8Array in the type definition
    const signedMessage = await walletA.signMessage(messageToSign);

    const messageHash = sha256(Buffer.from(messageToSign));

    const predicateData: TestPredicateInputs = [signedMessage, messageHash];

It needs to be as simple as this:

const messageToSign = 'gFUEL';
const signedMessage = await walletA.signMessage(messageToSign);
const predicateData = [signedMessage, message];

2. Passing in signed messages + messages to a predicate is a very common use for predicates, so this should be documented in the cookbook.

3. Helpers B256.from('xyz') and B512.from('xyz')