[Rust] Implement support for multipart file uploads for reqwest-async and reqwest-trait

[whirm]

I've updated the samples and related configs to enable useAsyncFileStream where necessary.

I've also sneaked in a fix for incorrect code generation, which I'd like someone to have a closer look:

https://github.com/OpenAPITools/openapi-generator/compare/master...whirm:feat/rust-request-multipart-file-uploads?expand=1#diff-560675cd0a01bd295018250fac1197e39c9cb662a6ccc8e15785d94f2b2ca769L423-R435

Now only primitive type parts get stringified with .to_string(), the rest are serialized to json. AFAICT that's the way to go.

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closes: #17822

@frol, @farcaller, @richardwhiuk, @paladinzh, @jacob-pro, @linxGnu Ready to review!

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[whirm]

Oops, I forgot I was on a different laptop! Fixed, thanks!