Add SignUp using UserAttributes class

[MilesLitteral]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

see #166

What is the new behavior?

users can use public async Task<Session> SignUp(SignUpType type, UserAttributes attributes, SignUpOptions options = null) to alternatively sign up as it seems the dictionary used by SignUp(sting username, string password) does not serialize to json correctly.

Additional context

This function can be used generally but is especially efficient when using this library with Unity.