IntoStyle for (&str, Option<T>)s

[luxalpa]

Is your feature request related to a problem? Please describe.
Conditionally setting css styles on an HTML element.

Describe the solution you'd like
In Leptos 0.6 it was possible to provide an Option<T> or an fn -> Option<T> to style:cssprop= attributes (i.e. .style(("prop", Some("value"))). I was somewhat depending on that feature, but it seems it was forgotten during the upgrade to 0.7.

Describe alternatives you've considered
The workaround right now is to use style overrides (like .unwrap_or("auto") or use just a single style attribute for all styles and render the HTML. However both of these approaches have downsides as they are not able to precisely add and remove the singular style from the element.

Additional context
I've tried doing he PR myself but actually the trait relationships with asyncOutput etc are really difficult and I'm not deep enough in the new leptos code base to understand it although I did spend a couple of hours trying to get something to work.

[luxalpa]

I didn't know these are different but I actually need IntoStyle for the reactive function that returns an Option<T> for my use-case.

[gbj]

Done for 0.8 in #3618