Create a file-based Upsert sink for testing internal components

There are a bunch of tests that in order to ensure correctness of their tested component rely on a Sink providing upserts. These tests (e.g. test-sql-client.sh) mostly use the ElasticsearchSink which is a lot of overhead. We want to provide a simple file-based upsert sink for Flink developers to test their components against. The sink should be very simple and is not supposed to be used in production scenarios but rather just to facilitate easier testing.