Stream multipart bodies from files

[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 is the first part of enabling streamed responses. This includes a ton of scaffolding, so the rest of the feature will be easy to implement. If a multipart form has a field whose value is a single-chunk template, and the root of the expression is a file() call it will be streamed to the server rather than loaded into memory. For example:

body:
  type: form_multipart
  data:
    file: "{{ file('slumber.png') }}"

Closes #582

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?

The tests rely on a PR I made to reqwest, which may or may not get merged. If it's not merged+released by the time this is ready to be published, I'll have to figure out another solution for the tests.

QA

How did you test this?

Lots of unit tests, tested the preview stuff in the TUI.

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Not yet, since the feature is incomplete
• 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