Add SEP-53 arbitrary message signing support

[tomerweller]

Summary

Implements SEP-53 (Stellar Arbitrary Message Signing) by adding two new methods to the Keypair class:

• signMessage(message): Signs an arbitrary message by prepending the SEP-53 prefix "Stellar Signed Message:\n", hashing with SHA-256, and signing with the ed25519 key.

• verifyMessage(message, signature): Verifies a SEP-53 signature by recomputing the hash and verifying against the public key.

Both methods accept string or Buffer inputs.

Use Cases

• Proving account ownership without on-chain transactions
• Off-chain attestations
• Application-specific authentication
• Wallet verification ("Connect Wallet" flows)

Implementation Details

• Uses the existing hash() function from hashing.js for SHA-256
• Uses the existing sign() and verify() primitives from signing.js
• Compatible with SEP-53 implementations in other SDKs (Python, Java, Freighter)

Example Usage

import { Keypair } from '@stellar/stellar-base';

// Sign a message
const kp = Keypair.fromSecret('SAKICEVQLYWGSOJS4WW7HZJWAHZVEEBS527LHK5V4MLJALYKICQCJXMW');
const signature = kp.signMessage('Hello, World!');

// Verify a signature
const publicKp = Keypair.fromPublicKey('GBXFXNDLV4LSWA4VB7YIL5GBD7BVNR22SGBTDKMO2SBZZHDXSKZYCP7L');
const isValid = publicKp.verifyMessage('Hello, World!', signature);

Closes #827

Test plan

• Added comprehensive tests for signMessage and verifyMessage
• Verified against SEP-53 test vectors from the specification
• Tests cover edge cases: empty messages, unicode, long messages, binary data
• Tests verify rejection of invalid signatures and wrong signers
• All 1132 tests pass (Node.js + browser environments)

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