Introduce custom Schema Registry deployment

With this MR, we'll eventually get rid of the Confluent Helm chart.

TODO:

• We still need to do some testing. Preferably with !285 (merged).
• Remove Confluent's Helm chart. (postponed to #352 (closed))

Fixes #355 (closed). We also complete one of the last open points for #265.

We decided to postpone #352 (closed) to next release, which will then also include #179 (closed), #74 (closed).