Add a setting to allow biometric-only access on IOS.

[ken0105]

• Make it possible to select storage that cannot be accessed by passcode in IOS. (iosBiometricOnly)
• To maintain backward compatibility, the default value is false.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[hpoul]

Shipped as darwinBiometricOnly in 5.0.1 — closing as done. Thanks.