Recover 'exo api' command registration removed by mistake

[kobajagi]

Description

When migrating to egoscale v3 we removed exo api command registration by mistake.
This PR adds it back and fixes it to use egoscale v3.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block)
• Testing

Testing

Before:

$ go run . api
error: unknown command "api" for "exo"
exit status 1

Now:

$ go run . api                                                          
Low-level Exoscale API calls -- don't use this unless you have to.

...