Create an importer for github-action-benchmark

[epompeii]

Create an import tool for https://github.com/benchmark-action/github-action-benchmark

Currently you either have to use the --backdate option or the Bencher Report POST API endpoint in order to import historical benchmark results.

Since making the switch is pretty common, add a bencher import CLI command that parses the github-action-benchmark JSON into backdated Reports using Bencher Metric Format (BMF).