Modular key support

[btoms20]

What
Redesign swift-libp2p-crypto to have modular key support

Why
The user / app developer should be able to decide which keys they would like to support in their app. Certain apps might not need Secp256k1 key support so we should be able to drop the Secp256k1 dependency via the package manager. Same for RSA keys, etc...