Add SLO checker for skipped records

Within KStreams it is possible that records are skipped or deleted. Skipped/deleted records are not correctly considered in the record lag metric. For this reason, we need an SLO checker to check the number of skipped records. To check this, we can use the metric dropped-records-rate|total.

This problem concerns the Python part (a new SLO checker script). The Kotlin related part is covered in #249 (closed) .