Add One-Time Pad Cipher implementation

[Nihhaar0002]

This PR adds an implementation of the One-Time Pad (OTP) cipher using XOR.

• Generates a random key using SecureRandom.
• Encrypts and decrypts using XOR.
• Validates matching key and message length.
• Includes JUnit tests for multiple cases.
• Adds comments explaining cryptographic limitations.

All tests passed successfully.