feat(auth): add support for managing custom headers in auth files

Closes #2457

sdk/cliproxy/auth/custom_headers.go

@@ -0,0 +1,68 @@
+package auth
+
+import "strings"
+
+func ExtractCustomHeadersFromMetadata(metadata map[string]any) map[string]string {
+	if len(metadata) == 0 {
+		return nil
+	}
+	raw, ok := metadata["headers"]
+	if !ok || raw == nil {
+		return nil
+	}
+
+	out := make(map[string]string)
+	switch headers := raw.(type) {
+	case map[string]string:
+		for key, value := range headers {
+			name := strings.TrimSpace(key)
+			if name == "" {
+				continue
+			}
+			val := strings.TrimSpace(value)
+			if val == "" {
+				continue
+			}
+			out[name] = val
+		}
+	case map[string]any:
+		for key, value := range headers {
+			name := strings.TrimSpace(key)
+			if name == "" {
+				continue
+			}
+			rawVal, ok := value.(string)
+			if !ok {
+				continue
+			}
+			val := strings.TrimSpace(rawVal)
+			if val == "" {
+				continue
+			}
+			out[name] = val
+		}
+	default:
+		return nil
+	}
+
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
+func ApplyCustomHeadersFromMetadata(auth *Auth) {
+	if auth == nil || len(auth.Metadata) == 0 {
+		return
+	}
+	headers := ExtractCustomHeadersFromMetadata(auth.Metadata)
+	if len(headers) == 0 {
+		return
+	}
+	if auth.Attributes == nil {
+		auth.Attributes = make(map[string]string)
+	}
+	for name, value := range headers {
+		auth.Attributes["header:"+name] = value
+	}
+}