Async await

[ctaggart]

Some help with OpenAPITools#4210

This may take a while. The next issue has something to do with async_trait:

42 | #[async_trait]
   | ^^^^^^^^^^^^^^ `std::rc::Rc<apis::configuration::Configuration>` cannot be shared between threads safely
   |
   = help: within `apis::tenant_api::TenantApiClient`, the trait `std::marker::Sync` is not implemented for `std::rc::Rc<apis::configuration::Configuration>`

I'll probably investigate more tomorrow. This can be merged as is if you want.

[ctaggart]

Looks like @srenauld solved this already in OpenAPITools#4175
https://github.com/OpenAPITools/openapi-generator/pull/4175/files#diff-7e03999fbf48b029588e3a360f6a27abR39

Due to the requirement that all futures should be Send, the reference-counting pointer is now an Arc

I'll port that change and try it out tomorrow.