| assetId | Custom assetId string. |
| spawnHandler | A method to use as a custom spawnhandler on clients. |
| unspawnHandler | A method to use as a custom un-spawnhandler on clients. |
This is an advanced spawning funciotn that registers a custom assetId with the UNET spawning system.
This can be used to register custom spawning methods for an assetId - instead of the usual method of registering spawning methods for a prefab. This should be used when no prefab exists for the spawned objects - such as when they are constructed dynamically at runtime from configuration data.