Add support for URL-encoded forms

[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.

Adds a new type of structured body, !form_urlencoded. The associated value is a key-value mapping. Example:

login: !request
  method: POST
  url: "{{host}}/anything/login"
  query:
    sudo: yes_please
    fast: no_thanks
  headers:
    Accept: application/json
  body: !form_urlencoded
    username: "{{username}}"
    password: "{{chains.password}}"

Closes #244

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 behavior of auto-header setting could potentially be confusing. This is just following the model set by the previous PR though (#251).

QA

How did you test this?

• Unit tests
• Manual testing in TUI

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