Allow BehaviorsAttribute inheritance

[swimmesberger]

By inheriting from the BehaviorsAttribute (it's currently sealed) we could combine multiple behaviors in e.g. a common behavior type for certain projects:

public sealed class DefaultBehaviorsAttribute: BehaviorsAttribute {
    public DefaultBehaviorsAttribute() : base(typeof(ValidationBehaviour<,>), typeof(TransactionBehavior<,>)) { }
}

// usage
[Handler]
[DefaultBehaviors]
internal static partial class AddDataSourceCommand {}