[Actionable Observability] Finalize SLO creation route handler

[kdelemme]

📝 Summary

Part of #137323
Depends on #139414, #139422, #139424, #139426

Once all previous tasks are done, we can wire everything together and start the Transformer so the slo data start being rollup.

✅ Acceptance Criteria

• The SLO definition is validated and stored
• The Transformer is created and started

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)