Improve require time by using `useAutoload: true`

[technicalpickles]

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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. https://github.com/OpenAPITools/openapi-generator/issues/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: https://github.com/OpenAPITools/openapi-generator/pull/13153

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