Add Scorecard Action to monitor Keras' security posture

[pnacht]

Hey, it's Pedro and I'm back (see #18762 and #18834) with another security suggestion:

I detected the issues fixed by those PRs by using Scorecard. It's a tool that scans a repository looking for settings that may make the project more vulnerable to supply-chain threats.

It is also available as the Scorecard Action, which continuously monitors the repository and adds security suggestions directly to the project's Security Panel. In doing so, it can let you know if something accidentally lowers the project's security posture.

I'll send a PR along with this issue adding the Action for you to take a look.

Note: Keras' current score of 7.8/10 places it in the top 2% of projects important to the open-source ecosystem!