[Feature] Add kibana_spaces support to elasticstack_kibana_synthetics_monitor

**Is your feature request related to a problem? Please describe.**

Synthetic monitors can only be assigned to a single Kibana space via the `space_id` attribute. The Kibana UI and API support sharing a monitor across multiple spaces using the `spaces` / `kibana_spaces` field (which maps to saved object `initialNamespaces`), but the Terraform provider doesn't expose this. This means users who need a monitor visible in multiple spaces must either duplicate the monitor or manage sharing out-of-band via API calls after `terraform apply`.

**Describe the resource you would like to have implemented.**

The Kibana Synthetics Monitor API ([Add Monitor API](https://www.elastic.co/guide/en/kibana/current/add-monitor-api.html)) accepts a `spaces` field (array of strings) in the request body. Internally this is stored as `ConfigKey.KIBANA_SPACES` and sets `initialNamespaces` on the saved object, allowing the monitor to appear in multiple Kibana spaces without duplication.

Relevant Kibana source:
- `x-pack/solutions/observability/plugins/synthetics/server/services/monitor_config_repository.ts` — uses `initialNamespaces: spaces` on saved object creation
- `x-pack/solutions/observability/plugins/synthetics/server/routes/monitor_cruds/edit_monitor.ts` — handles `KIBANA_SPACES` on update
- A wildcard value `["*"]` shares to all spaces

**Describe the solution you'd like**

Add an optional `kibana_spaces` attribute (list of strings) to `elasticstack_kibana_synthetics_monitor`:

```hcl
resource "elasticstack_kibana_synthetics_monitor" "my_monitor" {
name = "My Monitor"
space_id = "y"
kibana_spaces = ["y", "infrastructure"]
# ... other fields
}
```

When `kibana_spaces` is set, the provider should include the `spaces` field in the Kibana API request body. The `space_id` remains the "owner" space for the API endpoint path, while `kibana_spaces` controls visibility. A value of `["*"]` should share to all spaces.

**Describe alternatives you've considered**

- Duplicating monitors in each space (doubles cost, creates drift risk)
- Using a `null_resource` with `local-exec` to call the Kibana API after monitor creation (fragile, out-of-band state)
- Using `elasticstack_kibana_import_saved_objects` to copy the monitor SO to additional spaces (doesn't maintain sharing link)

**Additional context**

- Provider version tested: `0.14.4`
- Latest provider version checked: `0.15.2` (still missing this field)
- Kibana versions with this feature: 8.x+
- The Kibana UI exposes this under "Advanced > Kibana Spaces" in the monitor edit form

Locate the implementation and tests for the elasticstack_kibana_synthetics_monitor resource, then compare its request construction with the Kibana paths named in the issue: monitor_config_repository.ts and edit_monitor.ts. Done means an optional kibana_spaces list is accepted, sent as spaces while space_id remains the owner path, and supports the ["*"] wildcard.

Written by the indexing model from the issue text.