implement version-based hashing (SHA1/SHA3-256)

[alexsyvolap]

Currently, the SDK uses SHA-1 for all signature generations. According to the latest LiqPay API requirements, newer integration versions (specifically version: 7 and above) require the SHA3-256 hashing algorithm.

The current implementation does not support SHA3-256, which prevents using the SDK with the latest API features.

Proposed Solution
Update the signature generation logic to automatically select the hashing algorithm based on the version parameter provided in the request:

• Version < 7: Keep using sha1 (backward compatibility).
• Version >= 7: Use sha3-256.

Key Changes

• Refactor str_to_sign to accept an optional hashing algorithm.
• Update api and cnb_signature methods to determine the algorithm dynamically based on the version parameter.
• Add unit tests to verify boundary conditions (v6 vs v7) and ensure state isolation between requests.

Backward Compatibility
This change is fully backward compatible. Existing integrations using API v3 (or any version below 7) will continue to use SHA-1 by default