feat: add low-level request auth

Author: sdellaco

pkg/client/request.go

@@ -0,0 +1,20 @@
+package client
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"github.com/distribution/reference"
+	"net/http"
+)
+
+func (c *Client) RequestAuthenticate(req *http.Request, ref reference.Named) error {
+	authConfig := c.authProvider.AuthConfig(ref)
+	confBytes, err := json.Marshal(authConfig)
+	if err != nil {
+		return fmt.Errorf("failed to marshal auth config: %w", err)
+	}
+	encoded := base64.URLEncoding.EncodeToString(confBytes)
+	req.Header.Set("X-Registry-Auth", encoded)
+	return nil
+}