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]

Superseded by #997.

I renamed the fork branch to remove an internal project namespace from public PR metadata. GitHub closed this cross-fork PR during the branch rename, so I reopened the same change from the neutral branch:

• droplets: support usage-based backup policies #997

The replacement PR uses the same commit and branch name droplets/usage-based-backup-policy.