Convert from public JWK to PEM

[codeglobally]

Is your feature request related to a problem? Please describe.
Signatures using a public key provided via verification method in a controller document, need to be converted from JWK to it's PEM representation in order for the underlying crypto implementation to verify. Because there is no current mechanism for doing this conversion the verify service is unable to verify signatures based on controller documents

Describe the solution you'd like
Keypair.ts implements an abstract static method - fromJwk() - which is implemented by derived classes that converts the JWK into an instance of KeyPair which includes the public PEM.