Add multipart/form-data response builders to axum-extra

[zleyyij]

Closes #2624 (mine)
This pull request provides builders that enable multipart/form-data forms to be returned from handlers.

Motivation

multipart/form-data forms provide a way to include multiple types of data in a single response as key value pairs. You could include an image and a JSON, or any other combination of data.

As an example, this could be used for a pastebin, so that a single request to an axum backend could return metadata about a paste in a json, and the paste itself. The paste wouldn't need to conform to UTF-8, and could be sent separately from the json.

Solution

I added a MultipartForm struct that implements IntoResponse, and implementers can add parts to that form by defining Parts. I did not implement IntoResponse on the Multipart extractor, as advised in the original issue.

One dependency was added if the multipart feature is enabled. fastrand is used to generate boundaries, mirroring Reqwest's implementation.

Thank you for your time :)

[zleyyij]

I may need some help fixing those CI errors, I am unsure how to address them

[zleyyij]

Ok, I managed to fix all of those CI errors, thank you for your patience

[mladedav]

Thanks, for the PR I think the functionality looks good. I have not personally worked with multipart on this level so I don't know much of the technical details but I did comment on some of the other stuff.

[zleyyij]

I have commit ready to fix some of those minor issues, I just need to figure out how to handle mime type stuff from a public api level

[zleyyij]

I don't know how to do that, any advice would be appreciated

[zleyyij]

I gave up and just made any mime parsing happen internally, and the associated functions return an error.

I ran as much CI as I could with act locally, hopefully it passes

[zleyyij]

Is there anything more this PR needs?

[jplatte]

No, it's just waiting on my review. I've started looking at it now.

[zleyyij]

Those changes have been applied, thank you for your feedback :)