Document available column names for --nodes table

[dandrzejewski]

Now that the --nodes table can be configured with --show-fields, and soon sortable (see #740), it would be helpful to document the fields available.

We'd want to automate this as much as possible to minimize manual updates to the docs when the protobufs change.

At the very least, I'd like to add it to the CLI --help screen - perhaps --nodes --help. We can use the protobufs to enumerate everything and display it programmatically.

I'm aware that the protobufs aren't always the 'single source of truth' in this context, so I may need to do some refactoring here.

It's been noted on Discord that we need to keep in mind that downstream users of this library may be impacted by changes related to this.