backend: move LimitReadCloser to util package

The helper is only intended for usage by backend implementations.

internal/backend/util/limited_reader.go

@@ -0,0 +1,15 @@
+package util
+
+import "io"
+
+// LimitedReadCloser wraps io.LimitedReader and exposes the Close() method.
+type LimitedReadCloser struct {
+	io.Closer
+	io.LimitedReader
+}
+
+// LimitReadCloser returns a new reader wraps r in an io.LimitedReader, but also
+// exposes the Close() method.
+func LimitReadCloser(r io.ReadCloser, n int64) *LimitedReadCloser {
+	return &LimitedReadCloser{Closer: r, LimitedReader: io.LimitedReader{R: r, N: n}}
+}