events-api: consider metadata for identity relationships

[krzysztof]

Let's assume the state of objects (Identity groups) A and B that already exist in the database, and having their respective metadata tables (mA, mB). When an identity relationships comes in with, the groups A and B are merged into group C, and their metadata mA and mB is merged into mC. However, the metadata about the objects from the relationship itself is ignored, it should be three-way merged (mA, mB + the metadata from the payload). Metadata from the payload is currently ignored.