Platformize Envoy::Http::LowerCaseString.

header_api will be moved to third_party/quiche/common/balsa as part of the go/balsa4envoy project.

Refer to https://docs.google.com/document/d/16ZOQ4fOaHNUo9zAQDN8fLykqWAVuo5IvaUwpA_vpe6c/edit?resourcekey=0-swFCpsUOdlHwcc7Mn6-GRg for an analysis of alternatives.

PiperOrigin-RevId: 423319748

common/platform/api/quiche_lower_case_string.h

diff --git a/common/platform/api/quiche_lower_case_string.h b/common/platform/api/quiche_lower_case_string.h
new file mode 100644
index 0000000..4492b3c
--- /dev/null
+++ b/common/platform/api/quiche_lower_case_string.h
@@ -0,0 +1,16 @@
+// Copyright 2022 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef QUICHE_COMMON_PLATFORM_API_QUICHE_LOWER_CASE_STRING_H_
+#define QUICHE_COMMON_PLATFORM_API_QUICHE_LOWER_CASE_STRING_H_
+
+#include "net/quiche/common/platform/impl/quiche_lower_case_string_impl.h"
+
+namespace quiche {
+
+using QuicheLowerCaseString = QuicheLowerCaseStringImpl;
+
+}  // namespace quiche
+
+#endif  // QUICHE_COMMON_PLATFORM_API_QUICHE_LOWER_CASE_STRING_H_