publish `firmware-action` into GitHub action marketplace

[AtomicFS]

I think it would be great to publish the firmware-action on GitHub marketplace to get more exposure and some traction. I think I would also look more trustworthy and professional.

Currently blocked with:

9elements must accept the GitHub Marketplace Developer Agreement. Contact an org admin to accept it.

[AtomicFS]

/remind me to ping Christian on Aug 18

[github-actions]

@AtomicFS set a reminder for 2025-08-18

[github-actions]

👋 @AtomicFS, ping Christian