[Design] Fix for #975 - Implementing IActionConstraint and ActionMethodSelectorAt...

...tribute

IActionConstraint follows a provider model similar to filters. The
attributes that go on actions/controllers can be simple metadata markers,
the 'real' constraint is provided by a set of configurable providers. In
general the simplest thing to do is to be both an
IActionConstraintMetadata and IActionConstraint, and then the default
provider will take care of you.

IActionConstraint now has stages based on the Order property. Each group
of constraints with the same Order will run together on the set of
actions. This process is repeated for each value of Order until we run out
of actions or run out of constraints.

The IActionConstraint interface is beefier than the equivalent in legacy
MVC. This is to support cooperative coding between sets of constraints
that know about each other. See the changes in the sample, which implement
webapi-style overloading.
I think this would be better if its IEnumerable<IActionConstraint> in order to allow a user to bundle different constraints as part of the same attribute.

That way the constraints can have each a single responsibility and the factory just creates one or more of them.

The alternative would be to write a provider and do it there, but I think that writing a provider is much more complex than writing a factory and something that is more aligned with providing a constraint programatically in scenarios where you need context on the other actions defined in the application.

In my opinion, being able to provide more than one constraint should be enabled in the factory directly.