[FTR] Split config listing for serverless projects

[pheyos]

Summary

This PR splits the FTR config listing so we have separate entries for stateful confgs as well as configs per serverless project type.

Why do we need this?

• When running serverless tests on MKI, we don't use the local env setup that's defined in the configs files.
• Instead, we already have a project running and need to point tests to it.
• As a result, we need to know the list of configs that are suitable for each serverless project type

Change details

• The .buildkite/ftr_configs.yml config listing no longer has an enabled key
• Instead, it now has the keys enabled_stateful, enabled_serverless_search, enabled_serverless_security and enabled_serverless_observability
• Existing test configs have been moved to their corresponding new key
• Scripts that are reading the config listing have been adjusted accordingly

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 3bf9181

Metrics [docs]

✅ unchanged

History

• 💛 Build #174867 was flaky 5f8cd14
• 💛 Build #174714 was flaky 5ca96dc

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @pheyos

[pheyos]

Actually done in #187440