Add DigitalOcean provider support to autoscaler

[AJ0070]

This PR adds DigitalOcean as a supported autoscaler provider.

Changes

• add providers/digitalocean with:
  • provider flags
  • DigitalOcean API client
  • droplet create, list, and delete support
  • region, size, image, and SSH key resolution
  • pool-scoped tagging for managed droplets
• register digitalocean in cmd/woodpecker-autoscaler/main.go
• mark DigitalOcean as supported in README.md
• add unit tests for:
  • provider initialization
  • SSH key selection
  • droplet creation payloads
  • deployed agent listing
  • agent removal
  • tag sanitization

Billing behavior

DigitalOcean is treated as an hourly-rounded-up provider, so it uses the existing teardown behavior intended for hourly billing models.

Tested with this

• go test ./...
• go test -race ./providers/digitalocean

fixes #103

[AJ0070]

@6543 Can I get review on this?

[6543]

once i come to it ... or some other maintainer

[AJ0070]

@6543 Can you approve ci test?