Werk #14341: service_discovery: reimplementation of execute discovery mode endpoint

Previously, only the update detection mode provided the desired result, while all other modes either had no effect or behaved differently than described. This werk reimplements the discover_services action endpoint. In addition, the descriptions of certain modes are changed to better reflect the intended action. All modes return the same response schema as before expect the refresh option.

The refresh option starts a background job. The response is therefore different from the other mode responses and contains a redirect link to the current status of the running job. The job status can also be directly queried directly via the new endpoint /objects/service_discovery_run/{host_name} endpoint.

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