fix(NetworkClient): Defer ApplySpawnPayload until OnObjectSpawnFinished

[MrGadget1024]

This PR works when there are less than 20 objects, but fails when there are more than 20.

• Benchmark example fails with OnDeserialize warnings and disconnects with the error below.
• I checked the PR with several other examples and they're all fine, including Multiple Matches which spawns about 16 objects to client.
• subsequent spawning and unspawning via interest management is fine (Scene / Distance / Spatial - all fine).
• repeated StopClient - > StartClient in other examples is also fine.
• The warnings and errors clearly point to some sort of reader position pointer problem.
• SpawnMessage contains public ArraySegment<byte> payload; and that seems to be where the reader is getting lost, but SpawnMessage is a struct, and thus a value type, so I can't really make sense of why the payload isn't fully intact, unless it's not being written correctly.

if (message.payload.Count > 0)
{
    using (NetworkReaderPooled payloadReader = NetworkReaderPool.Get(message.payload))
    {
        identity.DeserializeClient(payloadReader, true);
    }
}

Reason: System.IO.EndOfStreamException: ReadBlittable<System.Byte> not enough data in buffer to read 1 bytes: [3F-10-03-0C-8E-04-63-41-00-00-00-00-8E-04-63 @ 18/15]
  at Mirror.NetworkReader.ReadBlittable[T] () [0x00047] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkReader.cs:142 
  at Mirror.NetworkReader.ReadByte () [0x00000] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkReader.cs:182 
  at Mirror.NetworkBehaviour.Deserialize (Mirror.NetworkReader reader, System.Boolean initialState) [0x00003] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkBehaviour.cs:1306 
  at Mirror.NetworkIdentity.DeserializeClient (Mirror.NetworkReader reader, System.Boolean initialState) [0x0002c] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkIdentity.cs:1128 
  at Mirror.NetworkClient.ApplySpawnPayload (Mirror.NetworkIdentity identity, Mirror.SpawnMessage message) [0x000f9] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkClient.cs:1150 
  at Mirror.NetworkClient.OnObjectSpawnFinished (Mirror.ObjectSpawnFinishedMessage _) [0x0005f] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkClient.cs:1328 
  at (wrapper delegate-invoke) System.Action`1[Mirror.ObjectSpawnFinishedMessage].invoke_void_T(Mirror.ObjectSpawnFinishedMessage)
  at Mirror.NetworkClient+<>c__DisplayClass61_0`1[T].<RegisterHandler>g__HandlerWrapped|0 (Mirror.NetworkConnection _, T value) [0x00000] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkClient.cs:543 
  at Mirror.NetworkMessages+<>c__DisplayClass9_0`2[T,C].<WrapHandler>g__Wrapped|0 (C conn, T msg, System.Int32 _) [0x00000] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkMessages.cs:206 
  at Mirror.NetworkMessages+<>c__DisplayClass8_0`2[T,C].<WrapHandler>b__0 (Mirror.NetworkConnection conn, Mirror.NetworkReader reader, System.Int32 channelId) [0x000cf] in C:\Unity\Mirror2023\Assets\Mirror\Core\NetworkMessages.cs:179

Original PR Description

==================
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.