[FEATURE] Improve relation handling

Improve relation in Content domain model. Relations that
are supported:

* One-to-Many relations
* Many-To-One relations

The patch also Introduces Repository Factory for keeping
independent instances of repositories according to their data type.

Change-Id: I70bad06abde73ea92b9026d20cd2ae5d01479090
Releases: 0.2.0
Fixes: #50831
Fixes: #50933