[Actionable Observability] Get Basic SLO information

[kdelemme]

📝 Summary

Part of #137323

Create a new endpoint to get an SLO details by its id, e.g. GET /observability/slos/{uuid} This API should return the basic details of the SLO:

• id
• name
• description
• time_window
• objective
• indicator
• created_at
• updated_at

[elasticmachine]

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