Create and include an analyzer that validates action method return types match the declared ActionResult<T>

Add an analyzer that validates the return types from an action method returning `ActionResult<T>`, e.g. don't let it return `ObjectResult` with a different T value, e.g. the following is invalid:

``` csharp
[HttpGet]
public ActionResult<IEnumerable<string>> Get()
{
    return new ObjectResult(new { Not = "Valid" });
}
```