Add command "Add ssh key to droplet" (Renewed, was not considered all around)

**What is the problem this feature would solve? Please describe.**
Being able to build from an API and assign a key after the fact without ever giving password access to a droplet

**Describe the solution you'd like**
doctl compute droplet-action add-ssh-key --shh-key

**Additional context**
When building a droplet through doctl we DONT EVER want to give password access to spotinstances, but we may also have to choose to "remove" the one and only assigned key, only programatically determined SSH keys, and be able to revoke will have to be a thing too of course.