Add option to show Wh/kWh instead of Joules

[ArneTR]

Originated from this issue: #106 (comment)

Idea: Since kWh is more known to people Eco-CI should allow displaying this unit instead of Joules.

[ArneTR]

The feature has been added to the badges: green-coding-solutions/green-metrics-tool#1061

Eco-CI now actually displays all badges by default in watt-hours

Pending is now an option to configure this.

Help wanted :)

[sufirehman]

Hi, I'd like to take this on since it's still open. I've added a configurable display_unit option (J / Wh / kWh, defaulting to J for backward compatibility) following the existing config patterns in this repo (action.yml input, ECO_CI_DISPLAY_UNIT env var for GitLab), applied consistently across the table header, per-measurement rows, and the summary totals in scripts/display_results.sh. I've manually verified the conversion (7200 J converts to 2.0000 Wh) and the existing badge logic is untouched, since that already defaults to Wh per #1061. Happy to open a PR if this approach looks right, or adjust based on feedback.