feat: Support formData fields ordering and reading them as they come in

[KristijanZic]

Description

There is a use case with multipart form data uploads where the uploading file is bundled together with the auth in the same form.
Now that creates an issue in Dart Frog where we can't authenticate the signature/token before the entire form data is cached on the server.
This means that we can't reject the request before caching the entire file ergo, somebody can send a couple of 1GB requests and overload the server.

It would be great if we could have something like reading the fields by order, get the auth ones first, verify them and then accept the rest or reject the request.

One would think that it would be the best to put the auth in the header but for some reason AWS S3 has it in the form data fields with the file. I don't know if there are some security implications etc. But anyone that wants to make an S3 compatible endpoint this will be a deal breaker issue.

Further context

• docs: FormData & Access Token #643 (comment)

[alestiago]

Hi @KristijanZic thanks for opening an issue!

Do you have any proposal in mind? Is there any other framework out there that had a solution that you liked for solving this use case?

I'm not sure if there exists a way to solve this use case where the authentication is within the body (cc: @wolfenrain ).

[wolfenrain]

We talked about this on the bloc discord, an idea to look into is using sync generators and iterables to only parse as much as the user requests instead of everything in one go

Theoretically it is possible but we need to verify if this is feasible and user-friendly