Storing information about resource consumers

[hqarawlus]

This expands on point 3b in International-Data-Spaces-Association/dsc-transfer#44.

Current Situation: Connectors can react to a ResourceUpdateMessage and update their locally saved copy of the resource. The information required to create a ResourceUpdateMessage must be manually provided, such as the affected resource ID, and most importantly, the URL of the consumer connector.

The aformenetioned information must be stored at the provider side in order to automate the process of sending the update messages. A suggestion here is that upon a successful resource request, the provider stores information about the consumer in a table, namely ResourceUUID + ConsumerURL. This information will then be accessed when a resource is updated to issue and send the relevant ResourceUpdateMessage.

Discussed with @jpieperbeck.