Operation request: Add custom alphabet support to Atbash and Affine ciphers

### Summary
Add the ability to specify different alphabets for use with the Affine and Atbash cipher operations.

### Possible solutions
The `editableOption` argument type could be useful. See '[To Base64](https://gchq.github.io/CyberChef/?recipe=%5B%7B%22op%22%3A%22To%20Base64%22%2C%22args%22%3A%5B%22A-Za-z0-9%2B%2F%3D%22%5D%7D%5D)' for an example of how this works. You could specify a few default alphabets (Latin, Hebrew etc.) whilst still allowing users to enter a custom alphabet of their choosing.