Don't add typescript index signatures on enum unions

In the generated types we often see the index signature: `[k: string]: unknown` on the enum union types, which in TS essentially allows for any additional property in the interfaces.

```ts
export type GovernanceDetailsForString =
| {
Monarchy: {
monarch: string
[k: string]: unknown
}
}
| {
External: {
governance_address: string
governance_type: string
[k: string]: unknown
}
}
```

```rs
/// Governance types
#[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq, JsonSchema)]
#[non_exhaustive]
pub enum GovernanceDetails {
/// A single address is admin
Monarchy {
/// The monarch's address
monarch: T,
},
/// An external governance source
External {
/// The external contract address
governance_address: T,
/// Governance type used for doing extra off-chain queries depending on the type.
governance_type: String,
},
}
```

No source file or test is named. Start by tracing the generator path that produces TypeScript enum union interfaces from the shown Rust enum, then compare generated output before and after the change. Done means enum union members no longer include the `[k: string]: unknown` index signature.

Vom Indexierungsmodell aus dem Issue-Text verfasst.