[TASK] Update NGSI-LD Bridge to merge content rather than update whole properties

[wagmarcel]

E.g. do not overwrite existing metadata such as UnitCode or other properties/relationships inside an attribute

Today: Update of properties can be done directly with information coming from ngsild.attributes kafka-channel.
However, merge API might require to provide the Type of object to update. That means you need to find out the type of the object from the entity definition. Today, every entity type is sent to a different Kafka topic e.g. Cutter is sent to ngsild.entities.Cutter topic which creates a problem when you want to access toe Entity table with a generic SQL command.
attributes=>SQL-CoreServices=>ngsild-update-channel=>ngsild-update-bridge
SQL expression cannot be generic over tables (e.g. you have to know which tables to access in advance)