fix(ios): Add support for reading __NSCFData key strings

[mold]

Fixes a crash caused by reading incorrectly encoded key strings on iOS.

Original PR description here: Crypho#154 (the previous maintainer chose not to merge it, hoping it will get merged this time 🤞)

Thanks for keeping this plugin alive!

[BorntraegerMarc]

I kind of agree with the original statement:

Since this is not very useful for anybody else, I am reluctant to merge. I would suggest you have a migration that checks the existing keys and migrates them before initialising secure storage.