Use external puffin viewer instead of integrating puffin_egui

[abey79]

Previously, we used the integrated puffin_egui as profiler UI. The advantage is that installing puffin_viewer wasn't necessary, but it means that we're stuck to whatever egui version puffin_egui is using, which is very annoying. This PR migrates to using puffin_http and the external puffin_viewer to lift this limitation.

Use cargo install puffin_viewer --locked to use the profiling feature.