Make `--state-version` in storage benchmark command self-descriptive

> Substrate `--dev` should use `V1` and Polkadot `V0`. Selecting the wrong version can corrupt the DB.

Given the help message above, I thought I should use `--state-version
V0` but it turned out `--state-version 0` was expected, which is
misleading. This commit fixes it by adding an enum for specifying
the StateVersion, making the rendered help message accurate and self-descriptive.