Support Spaces Key API

[lee-aaron]

Adds support for Spaces Key API based on digitalocean/openapi#958

[andrewsomething]

All these test are failing for me with 403s. If I update the names of the buckets with ones that already exist on my account, then they pass. We need to create the buckets used in the test as part of the configs. Take a look at the existing tests for Spaces buckets for examples, e.g.:

terraform-provider-digitalocean/digitalocean/spaces/resource_spaces_bucket_test.go

Lines 631 to 639 in 4ec164a

	func testAccDigitalOceanBucketConfig(name string) string {
	return fmt.Sprintf(`
	resource "digitalocean_spaces_bucket" "bucket" {
	name = "%s"
	acl = "public-read"
	region = "ams3"
	}
	`, name)
	}

The tests can be run with: make testacc TESTARGS='-run=TestAccDigitalOceanSpacesKey'