[Feature] Add kibana_spaces support to elasticstack_kibana_synthetics_monitor #3300

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) 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:

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