Werk #15551: downtime: make the show downtimes endpoint more explicit

Prior to this werk, it was unclear under what circumstances the endpoint returned service, host or both types of downtimes. In short, specifying the host_name parameter limited the response to only return host downtimes. This werk introduces the downtime_type parameter in order to make it easier to filter the results. This parameter is optional as the same result can be achieved through the query parameter only.

Since the endpoint response is now more specific, user scripts must potentially be adjusted accordingly: * when the host_name parameter was previously specified, the user must now specify the downtime_type parameter to "host" in order to receive the same downtimes as before

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