feat: Add RC4 stream cipher implementation

[harrish243]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• 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 PRs 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".

[diusazzad]

Since this is a cryptographic algorithm, adding a brief explanation of how RC4 works and its security status in the docstring would be very helpful for learners.

[harrish243]

@diusazzad
Hi! Thank you for the suggestion.
I have updated the module-level docstring in ciphers/rc4.py to include:

• A clear step-by-step explanation of how RC4 works (covering KSA and PRGA).
• A warning regarding its modern cryptographic insecurity (highlighting biases in initial bytes and its prohibition in secure protocols like TLS via RFC 7465).
  Please review the updated code, and let me know if there is anything else I should modify. Thanks!

[diusazzad]

Really appreciate you adding the RFC 7465 reference as well—that’s a great detail!

Everything looks good to go from my end