Allow runtime attribute values via static methods

Attribute values are limited to compile-time constant [attribute parameter types](https://github.com/dotnet/csharplang/blob/master/spec/attributes.md#attribute-parameter-types). This can be limiting when values need to be evaluated at runtime. For example, I might want to take my help text or license from some file, or I might want a non-constant default value, or I might want [dynamic options](https://github.com/commandlineparser/commandline/issues/9), etc.

One way around this would be a string pointer to a property/method which would supply the actual value:

```cs
class MyOptions
{
[Option('f', "foo", HelpTextProperty = nameof(FooHelpText), Default = nameof(FooDefault))]
public int Foo {get; set;}

private string FooHelpText {get { return File.ReadAllText("foo.txt"); } }
private int FooDefault {get { return Math.Pow(2,5); } }
}
```

Of course type validation would be performed (Default should have an assignable type to the property type, HelpTest should be a string, etc) for all such pointers (regardless of whether they are actually invoked, to fail fast in case of type inconsistency)

This is just one example, I believe this could be useful for many other attribute properties. To explicitly name the attributes currently defined in the library:
- `AssemblyLicenseAttribute` (don't really understand the multiple line thing BTW, why not just pass the full string including newlines)
- `AssemblyUsageAttribute` (same comment)
- `OptionAttribute`
- `ValueAttribute`
- `UsageAttribute`
- `VerbAttribute`

Commencez par examiner les définitions répertoriées de AssemblyLicenseAttribute, AssemblyUsageAttribute, OptionAttribute, ValueAttribute, UsageAttribute et VerbAttribute. Suivez la manière dont les valeurs de leurs attributs sont consommées, puis déterminez la portée de la résolution à l’exécution des méthodes ou propriétés et de la validation des types pour ces attributs. Le travail sera considéré comme terminé lorsqu’il inclura une conception arrêtée et une couverture des valeurs d’exécution prises en charge.

Rédigé par le modèle d'indexation à partir du texte de l'issue.