rustls: optionally use WebPKI roots to avoid panicking on Android & iOS

[ewilken]

Motivation

Using kube-rs on iOS, I came across a panic in the underlying hyper-rustls root cert initialization trying to access the native roots, which doesn't work on Android or iOS, according to this open issue.

Solution

My solution was to add a check to fall back to the WebPKI roots when on Android or iOS, which fixes the issue for me, but could make sense upstream, too.

Please let me know if this change makes sense to you or whether I'm missing anything. And thanks a lot for all the work you've done on the kube-rs ecosystem! 😊