[FEATURE]: Support nested schema/table input for defineRelations in Relations v2

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

I would like `defineRelations` to support a nested schema/table object shape for multi-schema projects, while preserving the same runtime behavior and type inference as the current flat `schema.table` shape used by the Relations v2 API. The current Relations v2 work is still evolving, and there are already related discussions around multi-schema usage and relation ergonomics.

Today, this works:
```ts
import { defineRelations } from "drizzle-orm";

export const relations = defineRelations(flatSchemas, ({ one, many, ...s }) => ({
"identity.person": {
birth_country: one["location.country"]({
from: s["identity.person"].birth_country_code,
to: s["location.country"].id,
}),
},
}));
```

but my suggestion would be like this:
```ts
const schemas = {
identity: { person },
location: { country },
}

defineRelationsMultiSchemas(schemas, ({ one, many, ...s }) => ({
identity: {
person: {
birth_country: one.location.country({
from: s.identity.person.birth_country_code,
to: s.location.country.id,
}),
},
},
}))
```

Start with the existing Relations v2 `defineRelations` API and its flat `schema.table` example, then compare it with the proposed nested schema/table shape for multi-schema projects. Done means the nested input is supported while preserving the current runtime behavior and type inference.

Written by the indexing model from the issue text.