Add RC4 (Rivest Cipher 4) stream cipher to ciphers/

[dipeshrayg]

Describe your change:

Implements the RC4 (Rivest Cipher 4) stream cipher in ciphers/rc4_cipher.py.

RC4 is a symmetric stream cipher historically used in SSL/TLS and WEP, now deprecated due to known statistical biases. This implementation covers:

• Key Scheduling Algorithm (KSA)
• Pseudo-Random Generation Algorithm (PRGA)
• Encrypt and decrypt functions with full round-trip verification

Added for educational purposes with a clear security warning.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate pull requests for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE_NUMBER".