cask/url: accept Class for using in Cask::URL#initialize

Homebrew#15754 fixed
`Cask::URL::DSL#initialize`, but we also need to fix
`Cask::URL#initialize` as that takes the same parameters.

Error:

```
Error: Cask 'mycask' definition is invalid: 'url' stanza failed
  with: Parameter 'using': Expected type T.nilable(Symbol), got type Class
  with value MyCustomDownloadStrategy
Caller:
  /opt/homebrew/Library/Homebrew/vendor/bundle/ruby/2.6.0/gems/sorbet-runtime-0.5.10461/lib/types/private/methods/call_validation.rb:113
```

Refs: Homebrew#15750

Library/Homebrew/cask/url.rb

@@ -148,7 +148,7 @@ def respond_to_missing?(method, include_all)
       params(
         uri:             T.nilable(T.any(URI::Generic, String)),
         verified:        T.nilable(String),
-        using:           T.nilable(Symbol),
+        using:           T.any(Class, Symbol, NilClass),
         tag:             T.nilable(String),
         branch:          T.nilable(String),
         revisions:       T.nilable(T::Array[String]),