Prepare PX Toolbox for a POC integration into the PX for WP Plugin

### Feature Description

An immediate priority is to set up the PX Toolbox with all that is needed to provide a proof-of-concept (POC) integration into the PX for WP Plugin.

This integration is meant to provide the CWV field data (if available) and lab data for any requested URL. To that end, we need to finalize the tooling integration far enough to retrieve the field data from the PageSpeed Insights API integration and the lab data from the Lighthouse data that is embedded within the former.

**Primar goals**
- [x] [Adapt tooling to include PSI API data outside of the embedded Lighthouse data](https://github.com/ampproject/px-toolbox-php/issues/17)
- [ ] [Add routing of the configuration to make the PSI API integration configurable from an external consumer](https://github.com/ampproject/px-toolbox-php/issues/18)

**Secondary goals**
- [ ] [Add logging to transport error state from individual tools to the final consumer](https://github.com/ampproject/px-toolbox-php/issues/19)
- [ ] [Add data formatting abstraction layer](https://github.com/ampproject/px-toolbox-php/issues/20)

Start by reading the linked issues #18, #19, and #20, then inspect the existing PSI API integration completed in #17. Done means the toolbox supports external configuration, transports individual-tool errors to the consumer, and provides the requested data formatting abstraction for the POC integration.

Written by the indexing model from the issue text.