Add SetTaskLocal service + layer

[jplatte]

This allows sharing state if the request and response types don't have a mechanism for that like https request extensions. It's also slightly more type-safe way than request extensions¹.

This could certainly use some more docs, though I'm really not good at writing docs. Maybe just an example would be sufficient?

Also if you want I can add this to ServiceBuilder as well. I don't know how well in sync ServiceBuilder and ServiceExt are right now, and personally I would probably use the Layer directly but definitely never ServiceBuilder 🤷🏼

¹ can't accidentally have code adding Arc<Foo> as an extension, and some other code that tries to access just Foo; can still try to access state that's not available (due to the layer being applied in the wrong place / conditionally / not at all)