[Scenes] Add build flags to enable/disable Default Scenes handlers.

[lpbeliveau-silabs]

As mentioned here, we currently have a config in the CHIPConfig to indicate whether or not we are relying on the default handlers to handle scenes actions on external clusters.

This needs to be linked to a build flag.

The suggestion is to have a : use_custom_scenes_handlers build flag so that default are being used unless developers intentionally want to implement custom handlers.