Integrate WPT GitHub action

[rviscomi]

https://github.com/WPO-Foundation/webpagetest-github-action

When someone creates a PR to change a custom metric, the GH action should automatically test it on WPT and comment on the PR with the output of the affected custom metric(s) and a link to the results.

It would also be nice to have a way to tell the GH action which URL(s) to use for testing and whether to use desktop and/or mobile, otherwise there should be sensible defaults. For example, the action could search the PR description for URLs and test config options like desktop/mobile.

[rviscomi]

The action can use our private WPT instance: https://webpagetest.httparchive.org/. If needed we can use a private key to limit the tests that can be run on that instance.