Added Polyalphabetic cipher

[aishvarya252]

Describe your change:

This pull request adds an implementation of the Polyalphabetic Cipher, a classical cryptographic technique that uses multiple substitution alphabets for encryption and decryption. The implementation includes both encryption and decryption functions, supporting cyclic use of a key to determine letter shifts. Non-alphabetic characters are preserved during processing.

Reference
For more details on the Polyalphabetic Cipher, refer to the Wikipedia page: https://en.wikipedia.org/wiki/Polyalphabetic_cipher

• Add an algorithm?

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 functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.