[Bug]: TS - "TypeError: Do not know how to serialize a BigInt" on JSON.stringify(notarization)

### Bug description

When trying to call `JSON.stringify()` on an `OnChainNotarization` object, you'll get a `TypeError: Do not know how to serialize a BigInt"`.

This is caused by `u64` values used for the Rust `OnChainNotarization` type:

```rs
pub struct OnChainNotarization {
...
/// The timestamp of the last state change.
pub last_state_change_at: u64,
/// The number of state changes.
pub state_version_count: u64,
...
}
```

Which is bound to `BigInt` values for `lastStateChangeAt` and `stateVersionCount` in the TS `OnChainNotarization` class.

Implementation:

In general u64 should be used internally to prevent having a [year 2038 problem](https://en.wikipedia.org/wiki/Year_2038_problem)

The equivalent `getter` implementations should return `string` values as this type is also used for BCS deserialization in the [TS BCS library](https://docs.iota.org/developer/ts-sdk/bcs#basic-types).

### Expected behavior

1. Calling `JSON.stringify()` on an `OnChainNotarization` object must work
2. Having a [year 2038 problem](https://en.wikipedia.org/wiki/Year_2038_problem) must be avoided

### Can the issue reliably be reproduced?

Yes

### Example repository

_No response_

### Steps to reproduce

-

### Language

Node.js

### Language Version

-

### Version

0.1.0

### Hardware specifications

-

### Additional info

_No response_