[Asset Inventory][GCP] Populate related.entity field for resources implemented so far

[kubasobon]

Summary of your changes

WIP Populate related.entity field for resources implemented so far.

Screenshot/Data

Related Issues

Closes https://github.com/elastic/security-team/issues/10392

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary README/documentation (if appropriate)

[mergify]

This pull request does not have a backport label. Could you fix it @kubasobon? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
  NOTE: backport-v8.x has been added to help with the transition to the new branch 8.x.