Add OpenSSF Scorecard workflow

[pnacht]

Hey, it's Pedro and I'm back (see #779 and #781) and I've got another security suggestion for netty-tcnative!

I'd like to suggest that the project add the OpenSSF Scorecard Action. The OpenSSF Scorecard runs a "meta-analysis" of the project's security posture, and the Action then populates the project's Security Panel with possible improvements to its security posture.

This data is fetched via GitHub's public API, and the project's current score can already be seen here. It's currently a 6.8/10, which puts netty-tcnative at the top 15% of relevant projects.

It was through Scorecard that I detected the issues fixed in #779 and #781. The Action would simply do the same thing, letting you know if there's anything you can do to improve netty-tcnative's security. The Security Panel notifications include not only the reasoning for each check's score (as seen in the link above), but also remediation steps.

If you're interested, let me know and I'll send a PR!

[pnacht]

Hey, friendly ping here, please let me know if you'd be interested in this PR. If not, feel free to close!