Add the ability to use a *static* key with [FromState]

[rynowak]

[FromState("foo", "bar")] assumes that you are going to look up the key bar in the route values to find the state store key to look up.

We should also provide the option to use this with a static key. If you want to use modeling binding but the key you want to look up is static, there's no good way to do this with [FromState].

[rynowak]

@halspang - let's come up with a design before coding this up