Add the ability to create a new user and assign them to a group immediately

Problem/Motivation

In our Okta setup, we needed to be able to create a user and assign their group immediately during creation. This feature would require the following pull request to get merged into the underlying API code here: https://github.com/dakkusingh/oktasdk-php/pull/3

Proposed resolution

Update the okta_api.users service to pass the group ids to the create method.