Allow using any type of private keys for auth

[yamalight]

trafficstars

As discussed in #188 - Exoframe currently only supports RSA & PEM private keys due to limitations of JWT library. But I think (need validation though) node.js does understand pretty much all existing private key formats, so I think we should support them as well. This can be done by using a lib like forge for PKI and just using JWT as a transport.

I am not 100% sure that's possible, but I think it would be good to explore it and implement if that turns out to be the case. 🤔

[yamalight]

Possibly relevant lib: https://github.com/PeculiarVentures/PKI.js

[yamalight]

Possibly we could use openssl via wasm for that

[yamalight]

This is done.