[Feature] elasticstack_kibana_slo for synthetic monitors

**Is your feature request related to a problem? Please describe.**
Its currently not possible to configure elasticstack_kibana_slo for synthetic monitors

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

Api extract from ui:


But apperently that indicator is not documented in https://www.elastic.co/docs/api/doc/kibana/v8/operation/operation-createsloop#operation-createsloop-body-application-json-elastic-api-version-2023-10-31-indicator

**Describe the solution you'd like**
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**Describe alternatives you've considered**
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**Additional context**
Add any other context or screenshots about the feature request here.

Start with the elasticstack_kibana_slo resource and the linked Kibana create-SLO API documentation, comparing the synthetic-monitor indicator payload with the provider's current SLO support. Confirm the required fields and expected Terraform behavior; done means synthetic monitor SLOs can be configured and managed through the resource.

Written by the indexing model from the issue text.