Add feature for method overloading exercise

See the [method-overloading exercise](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/method-overloading/.meta/design.md).

The following feature should be added:

There are three overloaded `Describe()` methods which each take a single parameter (character, destination and travel method. They are not addressed by this issue.

In addition to the above 3 overloads the instructions require that the code should allow:
- the caller to specify all 3 aspects in one call (character + destination + travel method).

- or the caller to specify just 2 aspects, character and destination, in which case `TravelMethod.Walking` should be used as the travel method.

There are 2 ways for the student to approach this. Firstly, to include one overload with 3 parameters and a separate one with 2 parameters. In fact the [boilerplate](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/method-overloading/MethodOverloading.cs) takes this approach.

```csharp
public static string Describe(Character character, Destination destination, TravelMethod travelMethod) {}
public static string Describe(Character character, Destination destination) {}
```

The alternative is to omit the 2 parameter method altogether and give the final (`TravelMethod`) parameter of the 3 parameter method a default value of `TravelMethod.Walking` as is shown in the [example](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/method-overloading/.meta/Example.cs) code.

```csharp
public static string Describe(Character character, Destination destination, TravelMethod travelMethod = TravelMethod.Walking) {}
```

The analyzer should report that the second approach, giving the parameter a default value, is preferable. This should also handle the case where named parameters are used.