Add show actions

[joaander]

Description

Add the subcommand show actions.

Proposed solution

Like show launchers and show cluster, show actions will write out the actions TOML. When passed --short (#21), it will list only the action names.

Additional context

Listing action names may be useful for command line autocomplete. The TOML output may be useful to debug problems as it confirms row's internal representation of an action (after resolving defaults).