Add `--header` to `benchmark overhead + storage` (#12204)

* Add header to 'overhead' command

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add header to 'storage' command

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update READMEs

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <info@kchr.de>

* .as_ref() and fmt

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <info@kchr.de>