Validate and update relationship for existing entities

[djs0109]

Describe the solution you'd like
Firstly, the relationship attributes are independent of the fact, whether the related entities really (still) exist (for NGSIv2).
However, productive workflows might be built upon the information provided by the relationship.

Secondly, relationship attributes can not be provisioned in entities that are created using keyValues format. By default "Text" will be used as the data type.

Therefore, by validating the attributes against "whether it is relationship attribute or not", more reliability and usability are provided.

Describe alternatives you've considered
Two functions can be added:

1. remove_invalid_relationship
2. add_valid_relationship

[github-actions]

Branch 379-Validate-and-update-relationship-for-existing-entities created!