chore(`RLNContract`/`RLNBaseContract`): use composition instead of inheritance

### Description
`RLNContract` class currently extends the `RLNBaseContract` class

### Proposed Solution / Feature Design
`RLNContract`, instead of extending `RLNBaseContract`, to initialise `RLNBaseContract` as a private class variable:
```ts
private baseContract: RLNBaseContract = new RLNBaseContract()`
```

### Notes
- https://github.com/waku-org/js-waku/pull/2341#discussion_r2089785645
