docs(samples): Add snippets for Apache Iceberg using Managed I/O

[VeronicaWasson]

Description

Add code snippets for reading and writing to Apache Iceberg using the Dataflow managed I/O transform.

Fixes doc bug: b/341806230 (part of the work for b/338063763)

Checklist

• [ ] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] Tests pass: mvn clean verify required
• [ ] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

- [dataflow/snippets/src/main/java/com/example/dataflow/ApacheIcebergRead.java:19](https://github.com/VeronicaWasson/java-docs-samples/blob/82be524623c67488addc7cba568d032df2470146/dataflow/snippets/src/main/java/com/example/dataflow/ApacheIcebergRead.java#L19), tag `dataflow_apache_iceberg_read` - [dataflow/snippets/src/main/java/com/example/dataflow/ApacheIcebergWrite.java:19](https://github.com/VeronicaWasson/java-docs-samples/blob/82be524623c67488addc7cba568d032df2470146/dataflow/snippets/src/main/java/com/example/dataflow/ApacheIcebergWrite.java#L19), tag `dataflow_apache_iceberg_write`

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[VeronicaWasson]

LGTM. A small request: please use two region tag declarations to exclude the main method from the code snippet.

The main method is the main body of the snippets though. Dataflow code tends to be longish because it builds a pipeline. e.g. see https://cloud.google.com/dataflow/docs/guides/write-to-pubsub#examples