[Feature Request]: Support for iceberg dynamic destination with varying schema

What would you like to happen?

Summary

Today, Beam supports dynamic destination routing for Iceberg, but the input PCollection still requires a single Beam schema. This makes it difficult to build generic ingestion pipelines where:

• new event types can appear dynamically
• each event type maps to a different Iceberg table
• each event type has its own schema

Problem

A common ingestion pattern is:

1. Read generic events from a streaming pipeline like Pub/Sub
2. Determine destination table from an attribute like event_type
3. Fetch the schema for that event type dynamically from a central schema store
4. Write each record to its corresponding Iceberg table

This works conceptually for table routing, but breaks down because PCollection has one schema for the entire collection. In practice, that means:

• dynamic table name is possible
• dynamic schema per destination is not
• newly introduced event types require pipeline changes

What is needed?

We need per-destination schema, similar to BigQuery DynamicDestinations.

Example Idea:

IcebergIO.write()
  .to(new DynamicDestinations<InputT, DestinationT>() {
      @Override
      public DestinationT getDestination(ValueInSingleWindow<InputT> element) { ... }

      @Override
      public String getTableIdentifier(DestinationT destination) { ... }

      @Override
      public org.apache.iceberg.Schema getSchema(DestinationT destination) { ... }
  })

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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