identity: created initial did:web server

* Adds create POST method (accepts pub key, redirects to new URL) * Adds read GET method (retrieves JWK from user's url) * Handles serialization of JWK * Testable/mockable UUID generation. Known issues: * Not actually compliant with did:web yet. We are supposed to return a did:document, but right now we return a Json Web Key * We don't do any auth on the endpoints yet * We don't use any persistent storage (no db) * We totally fake the returned pub key. I.e. we actually don't use the pub key from the POST at all yet.