Add Field::chunk

[itstristanb]

Added the function chunk for exum::extract's multipart Field to give a clearer option to use rather than try_next.

Motivation

I needed a way to stream large files from a frontend file uploader directly into a file. These files can be 10gb+ in size, and need to be paused at any time. Though this is doable with try_next, I felt that it was clearer from the user's side to have chunk.

Solution

By implementing the chunk function for axum::extract's multipart Field, readability of code and nicer access the underlying multer::Field's chunk funciton is allowed.

Change log

1. Added chunk function to multipart field
2. Changed to doc comment for clarification of usage, and changed &self to &mut self
3. Fixed formatting
4. Cleaned up nits and correct example to reflex best practices
5. Removed last unwrap

[rmxbalanque]

Looks good to me.

[davidpdrsn]

Makes sense to add as convenience even though it's the same as the Stream impl.

[itstristanb]

@davidpdrsn missed a formatting issue in the initial pr cuasing cli to fail. it is fixed now but needs further approval.

Thanks.

[davidpdrsn]

A few more nitpicks.

Wanna add a changelog entry as well?

[davidpdrsn]

I've cleaned up the docs just now so should be good now 😊

I've also noticed that we use unwraps here. Sorry if you saw that and based your original commit on that. I'm gonna remove those unwraps as well.

[davidpdrsn]

Thanks!