fix(NetworkClient): Defer ApplySpawnPayload until OnObjectSpawnFinished #3097

[MrGadget1024]

Virtually eliminates null refs with GO/NI/NB SyncVars by getting all objects into spawned dictionary before applying payloads and invoking hooks.

• No change to normal spawning, only initial spawn
• Uses and clears a static dictionary, so no garbage allocation

See ticket #3097

Test with PickupsDropsChilds example:

• Added SyncList to PickupsDropsChilds on player for everything player drops
• Added SyncVar to SceneObject for owner player object
• Started with built host client, dropped 3 things, then joined client from editor
• Depending on spawn order, either the SyncVar hook or the SyncList OnAdd handler would've thrown NRE's

public readonly SyncList<GameObject> drops = new SyncList<GameObject>();

public override void OnStartClient()
{
    drops.OnAdd += OnAdd;
    for (int i = 0; i < drops.Count; i++)
        drops.OnAdd?.Invoke(i);
}

void OnAdd(int index)
{
    Debug.Log($"OnAdd {index} {drops[index].name}");
}

[Command]
void CmdDropItem()
{
    . . .
    // set owner object
    sceneObject.owner = gameObject;
    . . .
    // Spawn the scene object on the network for all to see
    NetworkServer.Spawn(newSceneObject);

    drops.Add(newSceneObject);
}

[ReadOnly, SyncVar(hook = nameof(OnOwnerChanged))]
public GameObject owner;

void OnOwnerChanged(GameObject _, GameObject newOwner)
{
    Debug.Log($"SyncVar hook OnOwnerChanged {owner.name}", owner);
}

Before this PR

NullReferenceException: Object reference not set to an instance of an object
Mirror.Examples.PickupsDropsChilds.PickupsDropsChilds.OnAdd (System.Int32 index) (at Assets/Mirror/Examples/PickupsDropsChilds/Scripts/PickupsDropsChilds.cs:40)
Mirror.Examples.PickupsDropsChilds.PickupsDropsChilds.OnStartClient () (at Assets/Mirror/Examples/PickupsDropsChilds/Scripts/PickupsDropsChilds.cs:35)
Mirror.NetworkIdentity.OnStartClient () (at Assets/Mirror/Core/NetworkIdentity.cs:757)
UnityEngine.Debug:LogException(Exception, Object)
Mirror.NetworkIdentity:OnStartClient() (at Assets/Mirror/Core/NetworkIdentity.cs:761)
Mirror.NetworkClient:InvokeIdentityCallbacks(NetworkIdentity) (at Assets/Mirror/Core/NetworkClient.cs:1398)
Mirror.NetworkClient:BootstrapIdentity(NetworkIdentity) (at Assets/Mirror/Core/NetworkClient.cs:1371)

After the PR, all refs are valid.