[OAuth] Link oauth provider flow

[carmenlau]

• -[SKYContainer.auth linkOAuthProvider:(NSString*)providerID options:(NSDictionary*)options completion:(void(^)(NSError*, SKYUser*))]
  • Add a new auth provider to the user by going through the auth flow
  • providerID - A string that identify the login provider
  • options
    • scope
    • scheme - For redirect after authorization flow
  • This function returns a skygear user, and an access token of the new
    service, via delegate.
• -[SKYContainer.auth linkOAuthProvider:(NSString*)providerID, accessToken:(NSString*)accessToken completion:(void(^)(NSError*, SKYUser*))]
  • accessToken - Client calls this API if it already has an access token,
    skygear will try to login directly instead of going through the OAuth
    flow.