Use `api.models.Metadata`

[wslulciuc]

In #2853, we introduced api.models.Metadata, a utility class for OpenLineage.RunEvent, OpenLineage.JobEvent, and OpenLineage.DatasetEvent. Let's now use the class on the event write path POST /api/v1/lineage to allow for:

• Event validation (returning 400 on invalid events)
• Seamlessly access OpenLineage metadata: runID, job, datasets, facets, etc