feat: RESP3 notifications support & Hitless notifications handling [CAE-1088] & [CAE-1072] (#3418)

- Adds support for handling push notifications with RESP3. 
- Using this support adds handlers for hitless upgrades.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Hristo Temelski <hristo.temelski@redis.com>

push/registry.go

@@ -0,0 +1,61 @@
+package push
+
+import (
+	"sync"
+)
+
+// Registry manages push notification handlers
+type Registry struct {
+	mu        sync.RWMutex
+	handlers  map[string]NotificationHandler
+	protected map[string]bool
+}
+
+// NewRegistry creates a new push notification registry
+func NewRegistry() *Registry {
+	return &Registry{
+		handlers:  make(map[string]NotificationHandler),
+		protected: make(map[string]bool),
+	}
+}
+
+// RegisterHandler registers a handler for a specific push notification name
+func (r *Registry) RegisterHandler(pushNotificationName string, handler NotificationHandler, protected bool) error {
+	if handler == nil {
+		return ErrHandlerNil
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// Check if handler already exists
+	if _, exists := r.protected[pushNotificationName]; exists {
+		return ErrHandlerExists(pushNotificationName)
+	}
+
+	r.handlers[pushNotificationName] = handler
+	r.protected[pushNotificationName] = protected
+	return nil
+}
+
+// GetHandler returns the handler for a specific push notification name
+func (r *Registry) GetHandler(pushNotificationName string) NotificationHandler {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	return r.handlers[pushNotificationName]
+}
+
+// UnregisterHandler removes a handler for a specific push notification name
+func (r *Registry) UnregisterHandler(pushNotificationName string) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// Check if handler is protected
+	if protected, exists := r.protected[pushNotificationName]; exists && protected {
+		return ErrProtectedHandler(pushNotificationName)
+	}
+
+	delete(r.handlers, pushNotificationName)
+	delete(r.protected, pushNotificationName)
+	return nil
+}