feat: allow string RPC URL for provider configuration in verify and validate

[lx3133584]

Problem

SiweMessage.verify and SiweMessage.validate require an instantiated Ethers Provider object for EIP-1271 contract wallet verification. This forces consumers to take on a direct runtime dependency on Ethers even when they only need to specify an RPC endpoint URL.

Solution

• Expanded VerifyOpts.provider type to accept providers.Provider | string.
• Added getJsonRpcProvider(url) helper in ethersCompat.ts to construct a JsonRpcProvider across ethers v5 and v6.
• Updated checkContractWalletSignature in utils.ts to automatically instantiate a JsonRpcProvider when a string URL is supplied.

Testing

• Added unit tests in packages/siwe/lib/client.test.ts verifying that string RPC URLs are accepted and correctly instantiated as provider runners.
• Verified compilation and test suite execution pass cleanly.