docs(samples): Dynamic destinations in Iceberg I/O for Dataflow

[VeronicaWasson]

Description

Update the Apache Iceberg write sample to show the use of dynamic destinations, which is a new feature in Beam 2.60.

Relevant doc bug: b/371047621

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
• Please merge this PR for me once it is approved

[VeronicaWasson]

Closing because it turns out this will need to be a separate snippet, so I will need to rework it.