Add FileReader and FileBufReader utilities

The FileReader interface is the wrapper of io.Reader. It provides
the fs.FileInfo as well. The FileBufReader struct is the wrapper of
bufio.Reader, it also provides fs.FileInfo.

Signed-off-by: Benjamin Wang <wachao@vmware.com>

client/pkg/fileutil/filereader_test.go

@@ -0,0 +1,41 @@
+// Copyright 2022 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fileutil
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFileBufReader(t *testing.T) {
+	f, err := os.CreateTemp(t.TempDir(), "wal")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	fbr := NewFileBufReader(NewFileReader(f))
+
+	assert.Equal(t, fi.Name(), fbr.FileInfo().Name())
+	assert.Equal(t, fi.Size(), fbr.FileInfo().Size())
+	assert.Equal(t, fi.IsDir(), fbr.FileInfo().IsDir())
+	assert.Equal(t, fi.Mode(), fbr.FileInfo().Mode())
+	assert.Equal(t, fi.ModTime(), fbr.FileInfo().ModTime())
+}