wip: refactor offline download (#5331)

* base tool

* working: aria2

internal/offline_download/tools.go

@@ -0,0 +1,33 @@
+package offline_download
+
+import (
+	"fmt"
+
+	"github.com/alist-org/alist/v3/pkg/task"
+)
+
+var (
+	Tools           = make(ToolsManager)
+	DownTaskManager = task.NewTaskManager[string](3)
+)
+
+type ToolsManager map[string]Tool
+
+func (t ToolsManager) Get(name string) (Tool, error) {
+	if tool, ok := t[name]; ok {
+		return tool, nil
+	}
+	return nil, fmt.Errorf("tool %s not found", name)
+}
+
+func (t ToolsManager) Add(name string, tool Tool) {
+	t[name] = tool
+}
+
+func (t ToolsManager) Names() []string {
+	names := make([]string, 0, len(t))
+	for name := range t {
+		names = append(names, name)
+	}
+	return names
+}