PIP-476: Extensible Key_Shared Dispatcher Mechanism

[liangyepianzhou]

Related: PIP-474 PR #25706 discussion

Motivation

The community consensus reached during the PIP-474 discussion is that new dispatching strategies (such as Hot Key Overflow) should be developed and validated as standalone Dispatcher implementations, rather than being embedded into the core class as conditional branches. This requires addressing three prerequisites:

1. Method visibility: Core methods are private, preventing subclass overrides
2. Instantiation entry point: PersistentSubscription has hardcoded creation logic, making it impossible to load third-party implementations
3. Lifecycle awareness: Custom Dispatchers may manage additional resources (e.g., auxiliary ManagedLedgers) that need cleanup upon subscription deletion or topic unloading