Support validation of profiles extending other profiles

[Pfeil]

Is your feature request related to a problem? Please describe.
New profile shave to be copied to extend existing profiles. But the DTR allows defining an "extends" attribute, which we could consider (we most likely do not do it yet).

Describe the solution you'd like
Validation should consider the extends attribute and merge the definitions (preferred) or validate accordingly.

Describe alternatives you've considered
Let people copy profiles, as it is currently the case.

Additional context

• "Overriding" properties is then possible to some degree (e.g. change repeatability).
• Could also be implemented for types, if the DTR supports it.

[Pfeil]

Check before starting the implementation:

• Does the EOSC DTR allow inheritence? --> no
• Can we somehow use composition instead to avoid handling overriding issues?