[ftr] report all file sizes loaded on each page load

[spalger]

We would like to start tracking the size of all files loaded on every page load. We should probably create a new FTR config that loads Kibana in puppeteer and captures the size of all JS/CSS/IMG/etc files loaded.

The goal would be to understand the size of the page overhead, we should focus on just the assets which are loaded on every page view.

To avoid noise caused by application specific assets we should probably load a few apps (KP apps only since legacy apps will be gone soon) and then only record metrics for the assets that are loaded on all page views.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[tylersmalley]

This was resolved in #71319 by using the on disk bundle outputs.