Add support for Worker KV Namespace Bindings

[Johnee]

This MR relies on https://github.com/terraform-providers/terraform-provider-cloudflare/pull/443 (Implement Workers KV Namespaces), as the acceptance tests cannot be run without it.

Proposed syntax for implementing KV namespace bindings is:

# from aforementioned MR
resource "cloudflare_workers_kv_namespace" "example" {
 title = "example"
}

resource "cloudflare_worker_script" "example" {
  name = "example"
  content = "example"

  kv_namespace_binding {
    name = "example"
    namespace_id = cloudflare_workers_kv_namespace.example.id
  }
}

[patryk]

Thanks @Johnee !