Add webhook signature verification

[teesofttech]

Summary

Adds webhook signature verification support for #45.

Changes

• Adds TermiiWebhookSignature.Verify(...) for HMAC SHA512 verification.
• Supports raw byte payloads and UTF-8 string payloads.
• Accepts plain hex signatures and sha512= prefixed signatures.
• Rejects missing, malformed, invalid, and altered-payload signatures.
• Updates webhook README guidance and API coverage.

Validation

• dotnet test

Closes #45.

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