[rust] Support async file upload

[vincent-herlemont]

Adding upload support when generating async Rust code.

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[vincent-herlemont]

The function From.file for adding a file in multipart requests seems to require the stream feature of the reqwest crate.

[vincent-herlemont]

Mention @farcaller to review Rust part.

[wing328]

thanks for the PR

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

[farcaller]

Can we detect this at build-time?