Implement `meta` field for Users

In the [users](https://developer.wordpress.org/rest-api/reference/users/#schema-meta) endpoint, the type for `meta` field is not clear to us as [it seems](https://developer.wordpress.org/reference/functions/get_user_meta/) it might be one of several types.
```[tasklist]
### Tasks
- [ ] We should also implement update user test for this as the `meta` parameter is currently untested
```