exp: refactor workspace execution into an executor

[pmrowla]

Somewhat adjacent to #6440 (but not a direct requirement)

Currently workspace execution for experiments is handled directly in repo.experiments, while temp dir (and eventually ssh) execution are handled through the experiments.executor classes. While the executor classes are being reorganized to account for ssh/remote executors, it makes sense for us to refactor the workspace execution handling so that it is in line with the rest of the API.

[pmrowla]

This is something I'll probably get to once the basic remote execution scenarios are taken care of.