Investigate whether we need to use base64url encoding in FCL lib

[JohnGuilding]

In order to correctly validate the client challenge, we had to use base64url encoding over base64 encoding. The Fresh crypto lib seems to give conflicting options on what you can do. The implementation under src/ use base64 whereas the implementation that is actually tests against in the same repo uses base64url encoding.

We should investigate what encoding we should actually use.