Always use JsonWebKey for verification methods (alt 3)

[peacekeeper]

This is one out of 4 alternatives to address #12.

The alternatives are:

• Stick with Ed25519VerificationKey2020 and EcdsaSecp256k1VerificationKey2019 but fix some text and examples.
• Always use MultiKey for verification methods.
• Always use JsonWebKey for verification methods. <-- this
• Always use CesrKey for verification methods.

I suggest we use #31 to discuss which one is best, and keep the others clean.