Implement registration invariants

[mempirate]

Context

We need to make sure that validators cannot designate an operator from another data source if they themselves are not part of that data source. Otherwise, they could be free-riding on the collateral provided by those operators, without explicit permissions from the data source.

To mitigate this, we need 2 invariants:

• Reject registrations designating an operator that hasn't been registered yet
• Reject registrations that designate operators from a different data source