Return public key in JWK format (x,y)

[GabrielePrestifilippo]

As of now the generateECPair returns a base64 public key.
However, common FIDO UAF implementations usually expect a JWK format, so an x,y.

In the native code, we already have access to those. Would you (@mvayngrib) consider an MR to return that as well? If so, any preference on how to get them (separate method, as an optional argument, as an additional field in the return value)

P.S. may I ask why is there an RCT_EXPORT_METHOD(test) in the iOS code?