Add Vultr support

[andrewchilds]

Resolves #58. Unit and manual testing remain to be done. Intentionally not adding any new npm dependencies, so the provider code uses https.request to hit the Vultr API v2 endpoints directly.

This code was largely written by Claude 3.7 Sonnet.

Tasks:

• Unit tests
• Sign up for Vultr account
• Manual testing (help would be appreciated here!)

[gurgeous]

Excellent! I am new to overcast but I gave this a quick whirl. instances/regions/plans/snapshots seemed to work fine, though the keys in the output said undefined (screenshot below).

create came back with a 404. Same for images. I wanted to try sync but I wasn't sure how to use it. I tried passing both the cluster name and a host name, neither seemed to work?