Add a debug-mode only utility to connect all of DevTools to a provided DTD URI

[elliette]

Fixes #9655

Adds a checkbox to the DTD Tools panel (only shown in debug-mode) that lets the developer connect all of DevTools to that DTD URI:

This is then indicated on the DTD Tools screen:

[kenzieschmoll]

FYI we also have debugDtdUri here: https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/shared/development_helpers.dart#L36

Not sure how we want this new global to interact with that. Maybe it should be integrated so it is still possible to set the dtd uri programmatically, or maybe it should be removed.

[elliette]

Not sure how we want this new global to interact with that Maybe it should be integrated so it is still possible to set the dtd uri programmatically, or maybe it should be removed.

Oh I didn't realize that! The only reason I can think for keeping debugDtdUri is if we need DevTools to be connected to DTD on startup.