docs(abtests) v3 api

[Niamsy]

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🧭 What and Why

🎟 JIRA Ticket: OPTIM-1443 OPTIM-1456

Changes included:

• ADD abtests-v3 specs openapi
• POST /3/abtests allows more than 2 variants, a configuration, and requires a set of metrics to be retrieved during the A/B Test
• GET /3/abtests returns all the metrics requested during A/B test creation
• GET /3/abtests/{id} idem

🧪 Test