Add a benchmark command to poe

As mentioned in #5240, @snejus made a whole flow for making a benchmark of beets functions. It'd be good to formalise this somehow and add it to the poe command definitions, and maybe add it as a GH workflow as well.

Start with the benchmark flow described in issue #5240 and inspect the repository's poe command definitions. Decide how that flow should be formalized as a benchmark command and whether it should run in a GitHub Actions workflow; done means the command and any workflow are documented and usable.

Written by the indexing model from the issue text.