feat: Add universal signer to make multi-chain signatures easier

[skambalin]

• Add new way of signing messages using multi-chain signer interface
• Add support for new RPC method wallet_signMessage which allows signing messages for any Cere wallet accounts

const signerSolana = await wallet.getSigner({ type: 'solana' });
const signatureSolana = await signer.signMessage('Hello, world!');
   
console.log(signatureSolana);

const signerEvm = await wallet.getSigner({ type: 'ethereum' });
const signatureEvm = await signer.signMessage('Hello, world!');
   
console.log(signatureEvm);