Add @Sendable conformance to KeychainSwift class

[theoks]

I took a look at the KeychainSwift implementation and it seems (unless I'm missing something) that it's thread safe since it's using a NSLock for synchronization.

This means that it could conform to Sendable, thus making modern concurrency adoption easier.

[x-sheep]

KeychainSwift needs several breaking changes for a proper conformance to Sendable:

1. The config properties accessGroup and synchronizable should have the public setter removed
2. The lastResultCode property should either use the lock to be read publicly, or (better IMHO) be removed in favor of functions throwing an error.

This will allow an @unchecked Sendable conformance. The class can remain open and allow subclasses, as long as any implementors also make sure their classes are Sendable.