Support benchmarking from external repositories?

[tueda]

As far as I understand, this action currently supports two modes:

• Running benchmarks in the current repository and pushing the results to the GitHub Pages branch within the same repository.
• Running benchmarks in the current repository and pushing the results to a different repository.

It may be useful to have another mode:

• Running benchmarks in a different repository and pushing the results to the current repository.

This can be used with triggers such as cron or workflow_dispatch to benchmark code in an external repository.

A partial implementation that is sufficient for my use case (I use customSmallerIsBetter, so the action does not need to build or run benchmarks) is available here. An external repository can be provided by the ref-repository input (URL) to retrieve the commit metadata for the given ref.