Embed Flutter DevTools in Chrome DevTools

Copied from https://github.com/flutter/devtools/issues/2930, since this is in fact a Dart Debug Extension request.

Flutter DevTools embedded in Chrome DevTools is supported internally, but not externally. This would required updating Flutter's response headers to include the `origin-agent-cluster`.

See: https://web.dev/origin-agent-cluster/#why-browsers-can't-automatically-segregate-same-site-origins

Start by locating the code that produces Flutter's response headers for the Dart Debug Extension. Read the linked origin-agent-cluster explanation and verify how Chrome DevTools embedding is handled internally versus externally. Done means external Flutter DevTools embedding in Chrome DevTools works with the required response header.

Written by the indexing model from the issue text.