merge benchmark results and upload to GitHub Pages

[crazy-max]

fixes #13

Merge benchmarks, generate graphs and upload to GitHub Pages.

TODO

• ~Move ref_timestamp out of benchmark metrics (this is just used for sorting refs in graphs)~

Follow-ups

• Needs to review testconfig for metrics value based on its unit to make graphs relevant
• testutil.MergeBenchmarks should return an object that can be ingested directly by format output of merge command.
• Graphs are reset on GitHub Pages after pipeline invocation. We should upload them in a specific folder and have an index file referencing all runs.

See https://moby.github.io/buildkit-bench/