Replace SAMKeychain in CKSession.swift

[vishnuravi]

SAMKeychain is deprecated - can we replace this with https://github.com/evgenyneu/keychain-swift? Or is there another keychain manager we could use?

[PSchmiedmayer]

@vishnuravi keychain-swift seems like a well-maintained framework. KeychainAccess also seems like a viable option.

A more general consideration: We are only storing minimal information in the keychain anyways, and the API surface of all these packages is not that big anyway. Therefore, I would also consider it doable to directly rely on the Security framework provided by Apple. Their documentation improved over the last few years, and even though they don't offer a nice "Swifty" API, the few lines we would need to add also don't look too complicated: Adding a Password to the Keychain. This would make us independent of one additional dependency.

[vishnuravi]

This issue has been addressed by #171