Offer a multiselect plugin upgrade experience

[itowlson]

You can currently upgrade plugins via spin plugins upgrade foo or spin plugins upgrade --all.

We could also allow spin plugins upgrade without the name or flag to launch an interactive experience similar to the templates one, in which it displays a list of plugins and the user ticks which one to upgrade.

[lann]

I would vote for plain spin plugins upgrade to be equivalent to --all.

[itowlson]

Heh... that was what I originally did with templates (so to get the multiselect UI you had to do spin templates upgrade --ask). But other people preferred "ask by default and have a flag for --all". I'd be happy to go back if a wider sample of opinion says otherwise. But regardless, I think we should use the same default in both places.