Unify Handlers via Adapter Interfaces (Future-Proof)

[Extrys]

Please drop all related feedback into the source PR for having everything in the same place

Key improvements in this extended branch of PR Unity-Technologies#3430

• New interfaces
  • public INetworkPrefabInstanceHandlerSource: Common abstraction for all handler types, exposing a consistent adapter regardless of instantiation data
  • internal INetworkPrefabInstanceHandlerAdapter: Centralizes the instantiation flow under a single contract, removing the need for branching based on handler type

While INetworkPrefabInstanceHandlerAdapter remains internal, its design allows safe exposure for custom user-defined adapters if needed

• Adapter-based routing

  • All handler types (INetworkPrefabInstanceHandler, INetworkPrefabInstanceHandlerWithData<T>, future variants...) now expose an adapter via INetworkPrefabInstanceHandlerSource
  • Eliminates type checks and separate dictionaries by handler type
  • Unifies all instantiation logic: for example, AddHandler now operate via the unified interfaces
  • The adapter determines whether custom data is needed, removing branching and parallel data paths
  • The system operates entirely through the adapter interface, reducing complexity and maintenance overhead

• Compatibility adapters
  LegacyHandlerAdapter and PrefabInstanceHandlerWithDataAdapter wrap existing implementations without changing their internal behavior, for backwards compatibility