Add command to `coder ping` a workspace

`coder ping` is useful to diagnose connection problems to workspaces. We should add support to the plugin to invoke it on a workspace. If we have trouble connecting to a workspace, it would be nice to prompt the user to run this command.

`ping` output is lightly formatted text. For a first iteration we could just display the output verbatim, but we could also consider an enhanced version that gets structured JSON from the CLI and nicely formats the results.