[I/O] RisingWave: Generalize event handler for subscriptions

## Intro

@hlcianfagna contributed an excellent tutorial about using RisingWave with CrateDB the other day. Thank you so much!

- https://github.com/crate/cratedb-guide/pull/207

## Pitch

> It would be great if these statements could be created dynamically, so it becomes a generic solution.

## Details

https://github.com/crate/cratedb-guide/blob/7484e80e494948990ed832445576c3fa3d86a646/docs/integrate/etl/iceberg-risingwave.md?plain=1#L204-L219

## Thoughts

The ingredients read tremendously familiar like some routines in [commons-codec](https://github.com/crate/commons-codec).

_Originally posted by @amotl in https://github.com/crate/cratedb-guide/pull/207#discussion_r2128746079_