droplets: support usage-based backup policies

[theredspoon]

Summary

• add window_length_hours and retention_period_days to DropletBackupPolicyRequest
• include those fields in Droplet create, multi-create, and backup-policy action payloads when set
• omit empty weekday from backup-policy action payloads, which allows usage-based policies such as intra_daily_4h

Context

DigitalOcean supports newer usage-based Droplet backup policies such as:

{
  "plan": "intra_daily_4h",
  "hour": 0,
  "window_length_hours": 4,
  "retention_period_days": 7
}

godo can already read these fields through DropletBackupPolicyConfig, but DropletBackupPolicyRequest and the Droplet action helpers could only send plan, weekday, and hour.

This is needed by the Terraform/OpenTofu provider so it can model usage-based backup policies without out-of-band API calls. Related provider issue: digitalocean/terraform-provider-digitalocean#1525.

Tests

go test -run 'TestDroplets_CreateWithUsageBasedBackupPolicy|TestDroplets_CreateMultipleWithUsageBasedBackupPolicy' ./...
go test .
go test ./...

[theredspoon]

Small status update from downstream provider usage:

• This SDK change is the write-path prerequisite for Support usage-based Droplet backup policies terraform-provider-digitalocean#1525.
• I verified the raw Droplet action API accepts a usage-based backup policy when retention_period_days and window_length_hours are sent with plan: "intra_daily_4h".
• The policy reads back through doctl compute droplet backup-policies get as intra_daily_4h, hour 0, window 4, retention 7.
• I rebased this branch on current digitalocean/godo:main and go test -mod=mod . passes locally on the rebased branch.

I also tried the documented go test -mod=vendor . command from CONTRIBUTING.md, but the current digitalocean/godo repo does not include a populated vendor/ tree or vendor/modules.txt, so vendor mode exits before running tests.

Happy to adjust the SDK shape if maintainers prefer a different API surface. I kept this PR SDK-only so the provider can update godo through normal module tooling after release.