Fix host for ActiveStorage DiskService

Sidestep Google Cloud Storage's restrictive per-object rate limit.

Use an ActiveSupport::CurrentAttributes model to provide the host for service urls

`content_type` parameter is before `disposition` parameter.

This fixes following warnings:

```
test/models/variant_test.rb:11: warning: ambiguous first argument; put parentheses or a space even after `/' operator
lib/active_storage/attached/macros.rb:63: warning: instance variable @active_storage_attached_highlights not initialized
lib/active_storage/attached/macros.rb:25: warning: instance variable @active_storage_attached_avatar not initialized
```