Add tests for rust reqwest middleware client

[wing328]

Add tests for rust reqwest middleware client

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cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05) @jacob-pro (2022/10)

[jacob-pro]

I'm kind of confused why these need to be added individually like this?
All the crates exist within the same workspaces:
https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/rust/Cargo.toml
https://github.com/OpenAPITools/openapi-generator/blob/master/samples/server/petstore/rust-server/Cargo.toml
So if you run cargo test within the root of the workspace it will run all unit and integration tests in all the sub crates

[jacob-pro]

We should consider setting an MSRV here, or ideally having a matrix of both MSRV and latest. Similar to:
https://github.com/actix/actix-web/blob/45b77c68195eb933231290a09e9f6a0cca56aad8/.github/workflows/ci.yml#L21-L23

And then if we only run clippy on the MSRV so we avoid the issues here:
#13907

[wing328]

thanks for sharing that. agreed with the matrix of both MSRV and the latest.

let's see if anyone has time to do it in a separate PR after merging this one.