Perfbot commands for evaluating only size

Hi, I don't know if this is the appropriate spot to mention this request, please feel free to redirect me if not! For WG-binary-size purposes, it might be nice to have a perfbot command to evaluate a PR strictly for its binary size impact. For a lot of such PRs, it's inappropriate to run the entire perf infrastructure because we don't expect any appreciable cycle or wall time impact, and if we do we'd probably be fine following it up reactively, and we'd rather not take up extra queue time if we don't have to.

This is especially the case with "library-only" PRs which affect fairly fiddly bits of standard library code and touch their code size.

Review the existing perfbot command entry points and the WG-binary-size workflow first. Determine how a command can evaluate only a pull request's binary size impact without running the full performance infrastructure. Done means the command performs that focused evaluation and its behavior is covered by the project's relevant checks.