Add support for Droplet backup_policy schema (Daily backups)

[connorhsm]

Is your feature request related to a problem? Please describe.

Earlier this year, DO released daily backup frequency for Droplets. Before this, there existed an ability to configure weekly backups to occur on a specific day and time of the week.

None of this functionality is currently possible via this provider.

Describe the solution you'd like

Ability to configure the backup_policy parameter as specified by the DO API.

Describe alternatives you've considered

N/A

Additional context