Add DigitalOcean autoscaler provider

[growthstudioworks]

Summary

• add a DigitalOcean provider with CLI/env configuration flags
• resolve provider configuration on startup, including region, size, image region availability, and SSH keys
• use DigitalOcean-safe internal tags for pool/image ownership and list/remove droplets by pool tag
• wire the provider into the autoscaler binary and mark DigitalOcean complete in the roadmap

Closes #103.

Tests

• go test ./...