feat(digitalocean): add droplet provisioning support

[andrasbacsai]

Summary

• Adds DigitalOcean as a cloud provider for server creation, including token validation, region/size/image/SSH key discovery, and droplet provisioning.
• Adds API endpoints for DigitalOcean metadata and server creation, with OpenAPI documentation updates.
• Tracks DigitalOcean droplet IDs and statuses on servers, including validation checks, status refresh, power-on support, linking existing droplets, and optional droplet deletion.
• Adds UI support for DigitalOcean tokens, server creation flow, provider icon, and modal token refresh behavior.
• Adds tests covering DigitalOcean API flows, service behavior, droplet state refresh, deletion, token modal events, and model fillable fields.