feat: add decrypt_with_signature()

[ShubhamChndrvnshi]

tl;dr

• Adds decrypt_with_signature() and get_signature_for_round() to fetch Drand signatures once and reuse for multiple decryptions (1 API call instead of 10k for 10k messages)

changes

• Adds decrypt_with_signature(encrypted_data, signature_hex) in
  python_bindings.rs
• Adds get_signature_for_round(reveal_round) in python_bindings.rs
• Exports both functions with type hints in init.py
• Adds 3 tests in test_all_functions.py: test_get_signature_for_round,
  test_decrypt_with_signature, test_batch_decryption_optimization

why

• decrypt() fetches Drand signature from API on every call
• Decrypting N ciphertexts for same round = N redundant API calls for identical signature
• Batch operations need signature reuse to avoid bottleneck

testing

• pytest tests/test_all_functions.py passes (8/8 tests)
• ruff check . passes
• Batch test verifies 5 messages decrypt with single signature fetch

[basfroman]

@ShubhamChndrvnshi please update this PR with staging. I merged you previous PR.

[ShubhamChndrvnshi]

@ShubhamChndrvnshi please update this PR with staging. I merged you previous PR.

Awesome. Have taken a pull from staging to my pr. Again thanks for the review.
Let me know if I need to add any documentation changes to Readme.md

[basfroman]

@ShubhamChndrvnshi please update this PR with staging. I merged you previous PR.

Awesome. Have taken a pull from staging to my pr. Again thanks for the review. Let me know if I need to add any documentation changes to Readme.md

Everything is great. You described the docstrings well and added tests. That's fine. Thank you!