databases: Add backup-restore functionality to db create

[danaelhe]

Addresses #968

Adding API parity of create a new database cluster based on a backup of an existing cluster:

resource "digitalocean_database_cluster" "foobar" {
	name       = "%s"
	engine     = "pg"
	version    = "11"
	size       = "db-s-1vcpu-2gb"
	region     = "nyc1"
	node_count = 1
	tags       = ["production"]
  }
resource "digitalocean_database_cluster" "foobar_backup" {
	name       = "%s"
	engine     = "pg"
	version    = "11"
	size       = "db-s-1vcpu-2gb"
	region     = "nyc1"
	node_count = 1
	tags       = ["production"]
	
	backup_restore {
	  database_name  = "%s"
	}
	depends_on = [
		digitalocean_database_cluster.foobar
	  ]
  }

APICLI-2018

[andrewsomething]

Looks great! I left one comment inline re: the CheckDestroy piece and it needs a merge from main, but just about good to go.