Add support for multipart/form-data bodies

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

This adds first-class support for multipart/form-data bodies, similar to the existing application/x-www-form-urlencoded bodies. This one was a bit more complicated, mainly because reqwest forces the body to be a stream rather than binary blob when using the multipart builder. So we're forced to turn plain bytes into a stream. This is annoying now but leaves us in an ok spot for #256. An example:

chains:
  image:
    source: !file
      path: ./static/slumber.png
requests:
  upload_image: !request
    name: Upload Image
    method: POST
    url: "{{host}}/anything/image"
    body: !form_multipart
      filename: "logo.png"
      image: "{{chains.image}}"

Closes #243

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

I haven't tested this much so there's totally room for an edge case. Also large files are forced to be loaded entirely into memory, which could create issues for some people.

QA

How did you test this?

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked