Improve require time by using `useAutoload: true`

[technicalpickles]

Requiring xero-ruby can take quite awhile because of the number of classes and modules. This time adds up especially for apps that use multiple OpenAPI generated libraries. OpenAPITools/openapi-generator#12648 describes it well.

There is a fairly recently feature to switch requires to using autoload which don't actually load until a constant is used: OpenAPITools/openapi-generator#13153

I'm not familiar enough with OpenAPI to know where to configure this though or I'd put together a PR>