Enable dynamic completion of nouns

[weeclemans]

provides a quick list of IDs of the current rules

Type 'doctl compute firewall get', press 'TAB' and you get a list of the current IDs of rules

[scotchneat]

Hi @weeclemans, thanks for the suggestion. This request has actually come up in the past and there have even been attempts at implementing it (#712).

In discussing this with the team, we recognize this would be useful and also know that Cobra (the library we use to define the commands) has evolved and added support for dynamic completions.

Upgrading Cobra will require some effort to understand the impact of the changes. We'll keep this issue open to gather votes and get a sense of community interest. I'll also rename the issue to ensure this request catches a wider audience.