Is your feature request related to a problem? Please describe.
I don't see a way to declare a property as a foreign key (not as a mapped property à la @ManyToOne and friends), which for reasons is something I'd like to do (I'm using dataloader with GraphQL so I mostly am just using Mikro as an easier, type-safe-ish way to access the database).

Describe the solution you'd like
A way to declare a property as a foreign key without doing data mapping.

@Property({
  columnType: "int",
  references: "users(id)"
})
userId: number;

Alternatively, the ability to specify custom constraints (eg as SQL) would be good enough.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context about the feature request here.