Add subscription manager

app/subscription/documentfetcher/fetcher.go

@@ -0,0 +1,32 @@
+package documentfetcher
+
+import (
+	"context"
+
+	"github.com/v2fly/v2ray-core/v5/app/subscription"
+)
+
+//go:generate go run github.com/v2fly/v2ray-core/v5/common/errors/errorgen
+
+type FetcherOptions interface{}
+
+type Fetcher interface {
+	DownloadDocument(ctx context.Context, source *subscription.ImportSource, opts ...FetcherOptions) ([]byte, error)
+}
+
+var knownFetcher = make(map[string]Fetcher)
+
+func RegisterFetcher(name string, fetcher Fetcher) error {
+	if _, found := knownFetcher[name]; found {
+		return newError("fetcher ", name, " already registered")
+	}
+	knownFetcher[name] = fetcher
+	return nil
+}
+
+func GetFetcher(name string) (Fetcher, error) {
+	if fetcher, found := knownFetcher[name]; found {
+		return fetcher, nil
+	}
+	return nil, newError("fetcher ", name, " not found")
+}