Add ed25519 / OKP key type support

[sandergroen]

I had some problems when implementing a sign on via an OIDC IDP. They had a ed25519 OKP key type public_key in their jwks.json. When implementing a custom OmniAuth strategy config.jwks.map(&:to_key) raised an exception on:

json-jwt/lib/json/jwk.rb

Line 49 in 9160130

raise UnknownAlgorithm.new('Unknown Key Type')

We have found a way to work around the issue but it would be great if this library supports this algorithm as well.