[Feature Request] Export foreign key schema in Zod plugin

[brunobely]

Is your feature request related to a problem? Please describe.
When integrating ZenStack with TanStack DB, having access to the model + foreign keys is very helpful.

Describe the solution you'd like
Adding an export to this generated line of code, i.e., export const fkSchema = ...; instead of just const fkSchema = ...;

Describe alternatives you've considered
For now I'm using [ModelName]ScalarSchema.passthrough(), which isn't type-safe.