[feat] Allow auto configure nullable route values

[ikesnowy]

Is your feature request related to a problem? Please describe.

It is frustrating when registering route for query/command that contains multiple nullable parameters.

Describe the solution you'd like

public record ItemQuery(string? AppName, int ItemId) : IQuery<ItemDto>;

// this maps following route
// "apps/{appName}/items/{itemId:int}
// "apps/-/items/{itemId:int}"
app.MapQuery<ItemQuery>("apps/{appName}/items/{itemId:int}");

you may configure this behavior:

app.MapQuery<ItemQuery>("apps/{appName}/items/{itemId:int}", mapNullableRoute: true, patternWhenNull: "-");
app.MapQuery<ItemQuery>("apps/{appName}/items/{itemId:int}", mapNullableRoute: false);