Add asset inventory AWS RDS fetcher

[kubasobon]

Summary of your changes

Adds an AWS RDS instance fetcher.

Screenshot/Data

Related Issues

Closes https://github.com/elastic/security-team/issues/9083

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)

Introducing a new rule?

no

[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-skip has been added to this pull request.

[github-actions]

📊 Allure Report - 💚 No failures were reported.

Result	Count
🟥 Failed	0
🟩 Passed	359
⬜ Skipped	33

[oren-zohar]

Why is there database twice in the index name?

Actually the same question sands for the s3 as well @romulets

[oren-zohar]

It's also unclear to me what the convention is for the index name when using - and _, I thought - is saved for the namespace. @romulets

[kubasobon]

@oren-zohar The index name is constructed here

func generateIndex(a Asset) string {
	return fmt.Sprintf("asset_inventory_%s_%s_%s_%s", a.Category, a.SubCategory, a.Type, a.SubType)
}

Type changed to relational-database, so it will be a bit different now. Subdivision is based on this chart.