Sample: Transfer a blob from a background context to a page

**Goal of the demo**

Demonstrate how an extension can create a blob in the extension's service worker and transfer it to a page's main world.

**Suggested implementation**

Sketch of my initial implementation plan

1. Have a content script inject an iframe into a page
2. Run a script in the iframe to post a message back to the service worker using `navigator.serviceWorker.controller.postMessage()`
3. In the SW's message handler, generate a blob OR use `structuredClone()` to create a copy of a blob.
4. Use `event.source.postMessage(msg, [transferable])` to reply to the client and transfer the blob
5. In the iframe's message handler, use `window.parent.postMessage(msg, "", [transferable])` to transfer the data to the page

**Related links**

* https://github.com/w3c/webextensions/issues/293

**Notes**

Initial findings suggest that it's not possible to transfer variables across origins. I'm tentatively thinking that to work around this, we can use `URL.createObjectURL()` in the iframe and directly reference the object URL somewhere that's easily visible to the end user (e.g. canvas, Audio/Video element, etc.).