create a listcommands command

I was thinking out loud in https://github.com/mozilla-services/antenna/pull/1100#issuecomment-2457261580 about why I wrote socorro-cmd, but don't generally like it and don't really want to carry it across to all the services we maintain.

That got me thinking that maybe instead of having a script runner we have a command that lists the available commands.

All of our commands are of one of two groups:

1. installed with a Python package -- we can list these automatically because they're listed in the console_scripts entrypoint package metadata

   https://docs.python.org/3/library/importlib.metadata.html#importlib.metadata.entry_points

   >>> import importlib.metadata
   >>> [ep.name for ep in importlib.metadata.entry_points()["console_scripts"] if ep.value.startswith(("obs_common", "crashstats_tools", "honcho"))]
   ["honcho", "gcs_cli", "pubsub_cli", ...]
   

2. located in the /app/bin directory

Given that, we can write a command that lists existing commands without having to manually maintain the list of commands.

Example usage:

> make shell
app> listcommands
Available commands:

honcho   gcs_cli   pubsub_cli  ...

See service developer documentation for more details.

Maybe it lists it by source?:

Example usage:

> make shell
app> listcommands
obs-common commands:

gcs_cli   pubsub_cli   waitfor   ...

crashstats-tools commands:

fetch_data   supersearch   reprocess   ...

/app/bin commands:

db   es   ...

See developer documentation for more details.

It could even be something we add to obs-common and note in our docs.

If we built this script in obs-common, then we maintain it in one place, it gets installed everywhere, and wouldn't have to do much to maintain it.

Start by reviewing the proposed importlib.metadata console_scripts lookup and the commands available in /app/bin. Resolve whether the command belongs in obs-common and whether output should be grouped by source, then confirm that it lists installed commands and /app/bin commands without manual maintenance.