butler support for cmd reloader

butler currently only supports an http reloader. it'd be great to support a cmd reloader as well so that we can provide butler with a cli command and arguments to handle the reload.

Start by locating the existing HTTP reloader and the entry point that accepts reloader configuration. Trace how reload requests are handled, then determine how a command and its arguments can invoke the same behavior. Done means Butler can run the supplied CLI command to handle a reload.

Written by the indexing model from the issue text.