Make crate work when two crypto backends are selected

[DoumanAsh]

This defaults to using aws's crypto lib if both features are enabled to make sure jsonwebtoken can be used

Genenrally everything should be re-written to make backend pluggable like in rustls, but this is the most simplest approach to work around issue of using jsonwebtoken in complex dependency tree

[Keats]

There is a PR to implement something like rustls: #452

[DoumanAsh]

Didn't notice one, I suppose I will wait for it to be implemented in order to fix build

[Keats]

If you have time, please review as well, i'm not familiar with the pluggable backend pattern

[DoumanAsh]

Ok, I looked briefly and it is not in state to be ready merged, so I left some comments
But jsonwebtoken has legitimate problem with being dependency as libraries cannot use it without either crypto feature, so depending on library that depends on jsonwebtoken is very dangerous, if PR is going to drag on, I may create another PR to implement pluggable backend myself as alternative