Werk #16384: notification rule: allow for non builtin service levels

When configuring a notification rule via the Rest API, you could not set the value for "match_service_levels" to anything but the default service levels. This werk addresses this issue by now allowing any of the service levels configured to be used. This change mean that there is a change to the request schema. Previously, we accepeted the service level string value, whereas now we accept the integer value.

Previous schema

{"match_service_levels": {
    "state": "enabled",
    "value": {"from_level": "silver", "to_level": "gold"}
    }
}

New schema

{"match_service_levels: {
    "state": "enabled",
    "value": {"from_level": 10, "to_level": 20}
    }
}

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