[REQ] Remove `RC` wrapping for Rust client

Is your feature request related to a problem? Please describe.

The RC wrapping Configuration generated in clients makes it impossible to share the client between threads (implements !Send), not even if you wrap it with Arc + Mutex.

For underlying clients like reqwest, this is a shame because that client can be shared between threads.

Describe the solution you'd like

For the most part, once someone has created an API Client using the Configuration, we can probably assume they're not going to use it again. We can then simply consume it.

We can then simply just hold the configuration (and its client) instead of this:

https://github.com/OpenAPITools/openapi-generator/blob/f2c1bdbf6feed76ac24215f931981990c2dfdc1e/modules/openapi-generator/src/main/resources/rust/reqwest/client.mustache#L29

Describe alternatives you've considered

We can also wrap with Arc instead.

Additional context

Start with modules/openapi-generator/src/main/resources/rust/reqwest/client.mustache at the Configuration wrapping referenced in the issue, then trace how generated Rust reqwest clients receive and retain that configuration. Done means the generated client no longer uses the RC wrapping and can be shared between threads while retaining the underlying client and configuration.