DE-3424: Initial SDK implementation (WIP)

[gunnarmorling]

Hey @rmetzger and @nicoweidner, this is all still very much in flux, but here's a draft PR for the upcoming SDK. It's fundamentally working, see the test for an end-to-end example. Feedback welcome, just focus on the high-level aspects at this point, polishing will come later. Thanks!

In terms of the general design, the idea is to have wrappers around Flink's Kafka source and sink, DecodableStreamSource and DecodableStreamSource; they are used like so, i.e. like any other Flink source or sink:

    DecodableStreamSource<String> source =
        DecodableStreamSource.builder()
            .withStreamName("purchase-orders")
            .build();

The topic name mappings, bootstrap servers, and any other potentially needed info is injected via env vars. Users should only ever work with the API in co.decodable.sdk; the internal package is well, internal, and mostly deals with wrapping the Kafka source and sink types (which was a PITA, but looks like I could make it work in the end).

[rmetzger]

I realized half way through that this is a draft PR. Still posting the comments I have so far.

[gunnarmorling]

And applied. Thx for reviewing, @rmetzger!