fix(rust,client): use local_var prefix for configuration

[PiDelport]

• Tracking issue: [BUG][RUST] Twilio OpenAPI codegen errors #10421

The Rust reqwest client code breaks due to a variable name conflict when the model has a parameter named configuration.

This happens with several twilio-oai specs.

This PR works around it by adding a local local_var prefix to configuration and referencing that (similar to the other local variables), which solves the issue when using useSingleRequestParameter, at least.

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[wing328]

@PiDelport thanks for the fix, which looks good to me