Support to generate a PDF report

I found some open-source projects below. Please see them if you're interested in it.

* https://github.com/jung-kurt/gofpdf
* https://github.com/pdfcpu/pdfcpu

We could put the feature into a separate CLI which is only rendering a JSON to a PDF or HTML. Such as: `atest run --report json | atest-pdf-report -`