Split MZKeychainWrapper out of FxA Crate

[skhamis]

To get SPM working, we needed to bundle SwiftKeychainWrapper with FxA to get it working on iOS. This is really hacky and not an ideal situation.

Summary of problem: Since it’s a library we need as a depedency and so does FxiOS there ends up being two copies since SPM can’t correctly make the dep graph.

Potential solutions:

1. Remove all SwiftKeychain files and put them in iOS

• Since they are the only consumer, the consumer-specific files should be in iOS

2. Create a script to split out SwiftKeychainWrapper in iOS during archiving

┆Issue is synchronized with this Jira Task

[mhammond]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1866510

Change performed by the Move to Bugzilla add-on.