Allow Phlex::Turbo::Stream element to take arbitrary attributes

[marcoroth]

With the Introduction of Custom Turbo Stream Actions it's not mandatory to always just pass the action and target attributes on the <turbo-stream> element.

Since you can write actions which don't target any elements you also don't need to specify the target or targets attribute (right now with Phlex 0.5 you also wouldn't be able to pass over a targets attribute).

Additionally, you need to be able to pass any attribute to the <turbo-stream> element so you can use custom attributes in your custom action.

For more context: hotwired/turbo-rails#375 and hotwired/turbo-rails#373

[joeldrapper]

Makes sense. 👍