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

`[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]`.