Implement `ForeignKey`

[surister]

CrateDB does not support the foreign key constraint, but it seems that it can be implemented in django in the client side by using django-signals, where django would perform these checks by issuing extra queries; at the cost of performance.

[surister]

Alternatively we could just not implement it for now until a user truly needs it. This feature goes against CrateDB design but might be needed to integrate to existing infrastructure that expects these integrity checks.