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>

internal/pool/hooks.go

@@ -0,0 +1,114 @@
+package pool
+
+import (
+	"context"
+	"sync"
+)
+
+// PoolHook defines the interface for connection lifecycle hooks.
+type PoolHook interface {
+	// OnGet is called when a connection is retrieved from the pool.
+	// It can modify the connection or return an error to prevent its use.
+	// It has isNewConn flag to indicate if this is a new connection (rather than idle from the pool)
+	// The flag can be used for gathering metrics on pool hit/miss ratio.
+	OnGet(ctx context.Context, conn *Conn, isNewConn bool) error
+
+	// OnPut is called when a connection is returned to the pool.
+	// It returns whether the connection should be pooled and whether it should be removed.
+	OnPut(ctx context.Context, conn *Conn) (shouldPool bool, shouldRemove bool, err error)
+}
+
+// PoolHookManager manages multiple pool hooks.
+type PoolHookManager struct {
+	hooks   []PoolHook
+	hooksMu sync.RWMutex
+}
+
+// NewPoolHookManager creates a new pool hook manager.
+func NewPoolHookManager() *PoolHookManager {
+	return &PoolHookManager{
+		hooks: make([]PoolHook, 0),
+	}
+}
+
+// AddHook adds a pool hook to the manager.
+// Hooks are called in the order they were added.
+func (phm *PoolHookManager) AddHook(hook PoolHook) {
+	phm.hooksMu.Lock()
+	defer phm.hooksMu.Unlock()
+	phm.hooks = append(phm.hooks, hook)
+}
+
+// RemoveHook removes a pool hook from the manager.
+func (phm *PoolHookManager) RemoveHook(hook PoolHook) {
+	phm.hooksMu.Lock()
+	defer phm.hooksMu.Unlock()
+
+	for i, h := range phm.hooks {
+		if h == hook {
+			// Remove hook by swapping with last element and truncating
+			phm.hooks[i] = phm.hooks[len(phm.hooks)-1]
+			phm.hooks = phm.hooks[:len(phm.hooks)-1]
+			break
+		}
+	}
+}
+
+// ProcessOnGet calls all OnGet hooks in order.
+// If any hook returns an error, processing stops and the error is returned.
+func (phm *PoolHookManager) ProcessOnGet(ctx context.Context, conn *Conn, isNewConn bool) error {
+	phm.hooksMu.RLock()
+	defer phm.hooksMu.RUnlock()
+
+	for _, hook := range phm.hooks {
+		if err := hook.OnGet(ctx, conn, isNewConn); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ProcessOnPut calls all OnPut hooks in order.
+// The first hook that returns shouldRemove=true or shouldPool=false will stop processing.
+func (phm *PoolHookManager) ProcessOnPut(ctx context.Context, conn *Conn) (shouldPool bool, shouldRemove bool, err error) {
+	phm.hooksMu.RLock()
+	defer phm.hooksMu.RUnlock()
+
+	shouldPool = true // Default to pooling the connection
+
+	for _, hook := range phm.hooks {
+		hookShouldPool, hookShouldRemove, hookErr := hook.OnPut(ctx, conn)
+
+		if hookErr != nil {
+			return false, true, hookErr
+		}
+
+		// If any hook says to remove or not pool, respect that decision
+		if hookShouldRemove {
+			return false, true, nil
+		}
+
+		if !hookShouldPool {
+			shouldPool = false
+		}
+	}
+
+	return shouldPool, false, nil
+}
+
+// GetHookCount returns the number of registered hooks (for testing).
+func (phm *PoolHookManager) GetHookCount() int {
+	phm.hooksMu.RLock()
+	defer phm.hooksMu.RUnlock()
+	return len(phm.hooks)
+}
+
+// GetHooks returns a copy of all registered hooks.
+func (phm *PoolHookManager) GetHooks() []PoolHook {
+	phm.hooksMu.RLock()
+	defer phm.hooksMu.RUnlock()
+
+	hooks := make([]PoolHook, len(phm.hooks))
+	copy(hooks, phm.hooks)
+	return hooks
+}