support dynamic index for elasticsearch sink

[neverchanje]

Is your feature request related to a problem? Please describe.

The user wants to create one sink that writes to multiple indexes dynamically. The sink decides which index to write to based on a column.

Describe the solution you'd like

index in ES is like tables in other relational databases. It's strange to automatically export data to different tables within one sink. But it might be reasonable in the context of ES. Flink supports it and they call this feature "dynamic index" https://nightlies.apache.org/flink/flink-docs-master/docs/connectors/table/elasticsearch/#dynamic-index

The user doesn't need RisingWave to create indexes because indexes are prepared beforehand.

CREATE SINK s AS
SELECT '(' || a || ',' || b || ')' AS idx
WITH (
  index_column = 'idx'
)

My proposal (if we finally decide to work on it) is to use a index_column field to specify the column that indicates the destination "index".

Describe alternatives you've considered

No response

Additional context

No response