refactor: extract GlobalObjectIdHash generation from NetworkObject.OnValidate()

[TwoTenPvP]

Occasionally, we might want to generate GlobalObjectIdHash manually in an internal flow when we know exactly what we're doing.
Recently, there was a use-case where we wanted to treat NetworkObject instances instantiated at runtime as NetworkPrefabs to avoid creating more and more NetworkPrefab assets to get GlobalObjectIdHash being generated.
By allowing internal code to manually execute GlobalObjectIdHash generation, we're simulating build-time/creation-time GlobalObjectIdHash generation at runtime — and again, in that context, we know exactly what we're doing there so that is fine.