created and added rivest_cipher.py

[adityaxanand]

Describe your change: I have added a new algorithm, RC4 (Rivest Cipher 4), which is a stream cipher used for encryption and decryption of messages. The RC4 algorithm generates a pseudorandom keystream based on a secret key and then XORs this keystream with the plaintext to produce the ciphertext. This addition includes a complete implementation of the RC4 algorithm in Python, along with detailed comments and examples to explain its usage. I look forward to feedback and further improvements.

• [x] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [ ] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [x] 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".

[cclauss]

https://github.com/TheAlgorithms/Python/pull/10699/files