Use a generic function for filtering any API request

[lebogg]

Currently each API endpoint implementation doing some filtering, is implementing it on their on. See, e.g.,:

clouditor/service/orchestrator/assessment_results.go

Lines 101 to 116 in 107d432

	// Filtering the assessment results by
	// * cloud service ID
	// * compliant status
	// * metric ID
	if req.FilteredCloudServiceId != nil {
	query = append(query, "cloud_service_id = ?")
	args = append(args, req.GetFilteredCloudServiceId())
	}
	if req.FilteredCompliant != nil {
	query = append(query, "compliant = ?")
	args = append(args, req.GetFilteredCompliant())
	}
	if req.FilteredMetricId != nil {
	query = append(query, "metric_id IN ?")
	args = append(args, req.GetFilteredMetricId())
	}

Try to use a more generic function that depending on the request and its filter fields, append the args and conds slices