Open bodies in external viewer

[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 a primitive implementation of opening recipe/request/response bodies in an external viewer (AKA pager). It adds an action menu item titled "View Body" that opens a subprocess to run the pager. There are a lot of improvements I'd like to make:

• Support a keybinding for opening
• Run the viewer in an embedded terminal, instead of having it take over the entire window
• Support different viewers for different content types (default and json would be the only two options for now)

Closes #404

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?

This is a half-baked implementation, so there's the risk I need to make breaking changes in the future. I'm not too worried about that though, because the only thing that could really break is the config schema, and that's simple enough to keep backward compatible.

QA

How did you test this?

Manual testing. It's tough to write a unit test for and I am feeling supremely lazy.

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