[Shiv-70]

What does this PR do?

Adds Base64 text encoding and decoding functionality to the CLI.

• Adds base64 encode and base64 decode commands.
• Supports UTF-8 text, including Unicode characters.
• Handles invalid Base64 input and non-UTF-8 decoded data with a clear error.
• Adds unit tests for encoding, decoding, Unicode, empty input, invalid input, and CLI behavior.

Related issue

Closes #24

Checklist

• I commented on the issue first and was assigned
• I removed the xfail marker (if fixing a bug)
• I added or updated tests
• pytest passes locally
• This PR only addresses one issue