Add rc4 cipher

[QuantumSoham]

Summary

Added an RC4 (Rivest Cipher 4) stream cipher algorithm under ciphers/ directory.

• Implements KSA and PRGA phases.
• Encrypts and decrypts using XOR.
• Fully type hinted and documented.
• Includes docstrings with doctests.
• CLI support via argparse.

All style and type checks (pre-commit, ruff, mypy) have passed.

Issue

No linked issue. (New Algorithm)

[Charuvarthan]

Should this be tested using doctest?

[QuantumSoham]

Should this be tested using doctest?

no not doctest
run using
python rc4.py encrypt Key Plaintext
or python rc4.py decrypt Key bbf316e8d940af0ad3

[cclauss]

Closing tests_are_failing PRs to prepare for Hacktoberfest