Feature Request: Add SEP-53 message signing and verification to Keypair #1492

// Signing a message
const keypair = StellarBase.Keypair.fromSecret('S...');
const signature = keypair.signMessage('Hello, World!');
// Returns: Buffer containing 64-byte ed25519 signature

// Verifying a message
const publicKeypair = StellarBase.Keypair.fromPublicKey('G...');
const isValid = publicKeypair.verifyMessage('Hello, World!', signature);
// Returns: boolean

// Binary message support
const binaryMessage = Buffer.from([0x01, 0x02, 0x03]);
const sig = keypair.signMessage(binaryMessage);

const MESSAGE_PREFIX = Buffer.from('Stellar Signed Message:\n');

/**
 * Sign an arbitrary message according to SEP-53.
 * @param {string|Buffer} message - The message to sign
 * @returns {Buffer} 64-byte ed25519 signature
 * @see https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0053.md
 */
signMessage(message) {
  if (!this.canSign()) {
    throw new Error('Cannot sign: no secret key available');
  }
  const messageHash = this._calculateMessageHash(message);
  return this.sign(messageHash);
}

/**
 * Verify a SEP-53 signed message.
 * @param {string|Buffer} message - The original message
 * @param {Buffer} signature - The signature to verify
 * @returns {boolean} True if signature is valid
 */
verifyMessage(message, signature) {
  const messageHash = this._calculateMessageHash(message);
  return this.verify(messageHash, signature);
}

/**
 * Calculate the SEP-53 message hash.
 * @private
 */
_calculateMessageHash(message) {
  const messageBytes = typeof message === 'string' 
    ? Buffer.from(message, 'utf8') 
    : Buffer.from(message);
  const payload = Buffer.concat([MESSAGE_PREFIX, messageBytes]);
  return hash(payload); // Uses existing hash function (SHA-256)
}