Convert Oauth details to snake case

[bradydan]

Fix Xero API client to accept either camelCase format, etc, and convert it automatically to convert to snake case if it needs to.

See: https://github.com/consilience/csi-provisioning/issues/53

And see Xero docs:
https://developer.xero.com/documentation/auth-and-limits/oauth-issues

[judgej]

This is referring to passing the auth token details as an array into the Token object during instantiation.