Add file function to async_impl::multipart

[NaokiM03]

Related to #646

Add file function to async_impl::multipart::{Form, Part}.

Notes

Since From<File> for Body was added in #1360, I decided to use the stream function that uses it internally.

This comment says the backend did not support chunked requests, but Transfer-Encoding: chunked should be accepted for HTTP/1.1.

[Feriixu]

@NaokiM03 Are you still developing this? I would very much like this feature merged.

[NaokiM03]

@Feriixu

No, this pull request is open, not draft. This means that the implementation I envisioned has already been done and is not under development.

At the time I wanted this feature, but unfortunately I got no response. So I don't even know if this code is correct.
It would be better to ask one of the committers than to tell me.

[seanmonstar]

Sorry to have let this drop! The code looks excellent to me, starting CI, and assuming all is well, I'd be happy to merge.

[NaokiM03]

The test is down and I will try to fix it.
https://github.com/seanmonstar/reqwest/actions/runs/10588215279/job/29476515082?pr=2106

[NaokiM03]

@seanmonstar
Could you approve again?

Fixed an error that was occurring in CI. 350733f
Sorry, I ran all the tests again and found that the doc test was failing, so I fixed it. 24b6270

Sorry to have let this drop!

No problem at all. I understand that you are busy and I appreciate your contribution to the Rust ecosystem because I like Rust.