[droplets]: add support for droplet backup policy

[loosla]

This PR adds droplet backup policy to doctl.

For droplets on:

• droplet create
• get a policy for a droplet
• list policies for all existing droplets
• list policies for all supported droplets

For droplet actions on:

• enable backups (now you can add a policy with a flag --backup-policy)
• change backup policy

Thanks 🦈

[andrewsomething]

Looking good. Left a few comments in-line from my first pass review. Would be great to get some integration-style tests as well.

Thinking about the overall structure, it might make sense to group these new commands together under a backup-policies sub command. Something like:

doctl compute droplet backup-policies get
doctl compute droplet backup-policies list
doctl compute droplet backup-policies list-supported

It feels a bit cleaner to me. What do you think?

[andrewsomething]

🙌 LGTM!