Add feature to set Podium context headers on the HTTP request

[wkillerud]

We're working on some things at the layout level which passes on certain HTTP headers to its podlets. For developers of layouts it's useful to be able to set and change these HTTP headers in the developer tools.

We want to have a new panel in the devtools that sits next to the existing one, dedicated to developing layouts. Think how the React dev tools have two panels for different tasks.

Research how an extension can access and set headers on outgoing requests.

[wkillerud]

We need to do work with the webRequest feature.

It requires we list a new permission webRequest. Depending on how , and may need to ask for that permission on all URLs unfortunately, unless we want to limit the feature to localhost.

Some reading material:

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest
• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/onBeforeSendHeaders
• Example extension: https://github.com/mdn/webextensions-examples/tree/main/user-agent-rewriter (see manifest.json and background.js in particular)