generic: return content length alongside body

This increases efficiency for bigger response bodies.

Signed-off-by: Yonah <contrib@gensokyo.uk>

cmd/msrfetch/net.go

@@ -10,12 +10,12 @@ type netDirect struct {
 	c http.Client
 }
 
-func (n *netDirect) Get(ctx context.Context, url string) (io.ReadCloser, error) {
+func (n *netDirect) Get(ctx context.Context, url string) (io.ReadCloser, int64, error) {
 	var resp *http.Response
 	if req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil); err != nil {
-		return nil, err
+		return nil, -2, err
 	} else if resp, err = n.c.Do(req); err != nil {
-		return nil, err
+		return nil, -2, err
 	}
-	return resp.Body, nil
+	return resp.Body, resp.ContentLength, nil
 }