Use active help for list subcommand

[winder]

Problem

Using active help would be really helpful for exploring plugins with the list command: https://github.com/spf13/cobra/blob/main/active_help.md#adding-active-help-messages

Solution

Some proposals for the active help output:

$ conduit list [tab]
Provide one of the plugin types and plugin name to get a sample config.

<conduit list output>

$ conduit list exporter [tab]
Provide one of the exporter plugins to get a sample configuration.

<conduit list exporter output>