feat(hitless): Introduce handlers for hitless upgrades

This commit includes all the work on hitless upgrades with the addition
of:

- Pubsub Pool
- Examples
- Refactor of push
- Refactor of pool (using atomics for most things)
- Introducing of hooks in pool

commands.go

@@ -193,6 +193,7 @@ type Cmdable interface {
 	ClientID(ctx context.Context) *IntCmd
 	ClientUnblock(ctx context.Context, id int64) *IntCmd
 	ClientUnblockWithError(ctx context.Context, id int64) *IntCmd
+	ClientMaintNotifications(ctx context.Context, enabled bool, endpointType string) *StatusCmd
 	ConfigGet(ctx context.Context, parameter string) *MapStringStringCmd
 	ConfigResetStat(ctx context.Context) *StatusCmd
 	ConfigSet(ctx context.Context, parameter, value string) *StatusCmd
@@ -518,6 +519,23 @@ func (c cmdable) ClientInfo(ctx context.Context) *ClientInfoCmd {
 	return cmd
 }
 
+// ClientMaintNotifications enables or disables maintenance notifications for hitless upgrades.
+// When enabled, the client will receive push notifications about Redis maintenance events.
+func (c cmdable) ClientMaintNotifications(ctx context.Context, enabled bool, endpointType string) *StatusCmd {
+	args := []interface{}{"client", "maint_notifications"}
+	if enabled {
+		if endpointType == "" {
+			endpointType = "none"
+		}
+		args = append(args, "on", "moving-endpoint-type", endpointType)
+	} else {
+		args = append(args, "off")
+	}
+	cmd := NewStatusCmd(ctx, args...)
+	_ = c(ctx, cmd)
+	return cmd
+}
+
 // ------------------------------------------------------------------------------------------------
 
 func (c cmdable) ConfigGet(ctx context.Context, parameter string) *MapStringStringCmd {