Writes gnu extended header: gnu_long_name or gnu_long_link.
fn writeExtendedHeader(w: *Writer, typeflag: Header.FileType, buffers: []const []const u8) Error!void
fn writeExtendedHeader(w: *Writer, typeflag: Header.FileType, buffers: []const []const u8) Error!void {
var len: usize = 0;
for (buffers) |buf| len += buf.len;
var header: Header = .init(typeflag);
try header.setSize(len);
try header.write(w.underlying_writer);
for (buffers) |buf|
try w.underlying_writer.writeAll(buf);
try w.writePadding(len);
}