NetworkObjectPool feature parity with the version in the Unity Multiplayer Samples Boss Room sample project

[11BelowStudio]

Basically it now works just like the 'official' version.

• It's now a NetworkBehaviour, not a MonoBehaviour
• It's a singleton
• It uses the NetworkManager.Singleton instead of a m_NetworkManager
• It initializes the pool in OnNetworkSpawn instead of in Awake

[unity-cla-assistant]

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