feat: add allKeys api

[Badisi]

Storage implementations usually offers a way to iterate over all the entries in the storage :

• localStorage/sessionStorage : you can combine .length and .key(index)
• cordova-plugin-secure-storage-echo : you can use .keys()

This benefits in being able to only remove the entries you were responsible for (ie. looking for entries with prefixed keys) and not remove the whole storage.

-> For iOS, SwiftKeychainWrapper already implements an .allKeys api (missing in their doc).
-> For Android, I haven't checked yet.

[martinkasa]

Added in v0.5.0 by PR #27