Fix webhook signature verification for multi-byte payloads and add byte[] overload

[IamPritamAcharya]

Fixes #351

Problem

Webhook signature verification could fail for payloads containing special or multi-byte characters due to use of platform default charset.

Fix

• Use UTF-8 encoding explicitly for hashing
• Add overloaded method to accept raw byte[] payload for safer verification

Changes

• Added verifyWebhookSignature(byte[] payload, ...)
• Updated hashing logic to use StandardCharsets.UTF_8
• Ensured backward compatibility by delegating the String-based method to the byte[] implementation

Tests

• Added test case with UTF-8 payload (emoji and multi-byte characters)
• Verified using:
  mvn -Dtest=UtilsTest test

Notes

• This change is backward compatible
• Full test suite failures are due to existing Mockito + OkHttp mocking issues unrelated to this change

[IamPritamAcharya]

Hi @ankitdas13 @HarshLileshShah
Just following up on this PR. This fixes webhook verification issues with multi-byte payloads and adds a raw byte[] overload for correctness.
Would really appreciate a review when you get time.