CaptiveCrypto for iOS native code refactor

[hawkinsji]

Is your feature request related to a problem? Please describe.

The native code in the CaptiveCrypto sample app for iOS is all in one big file.
https://github.com/vmware/captive-web-view/blob/main/forApple/CaptiveCrypto/CaptiveCrypto/StoredKey.swift

Describe the solution you'd like

The code should be in several smaller files, perhaps making use of a Swift enumeration as a namespace.

This type of solution was already implemented for #28 which was the same issue in the code for Android.

Describe alternatives you've considered

No response

Additional context

No response