feat: prevent to run with pull_request_target trigger unless skip-trigger-verification config is provided

[dancamma]

Closes #498

This PR prevent the action to run with pull_request_target unless the user explicitly provide a skip-trigger-verification config. For more details, see #355

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[simoneb]

Folks we need to converge on an agreement before we go ahead with any implementation. At the moment I'm not sure we reached one yet.

[dancamma]

Hi @simoneb, I opened this for reference in the thread of #500. I'm converting this to a draft.

[simoneb]

Closing this as there is no consensus