docs(samples): Add Dataflow snippet for reading from Cloud Storage

[VeronicaWasson]

Description

Adds a snippet that shows how to reading from a Cloud Storage file into Dataflow.

Snippet bug: b/340947448 Related doc bug: b/337851367

I structured this snippet similarly to the KafkaRead sample, where the snippet includes the code to create the pipeline, but not omits the pipeline options class and running the pipeline.

Also fixed some integration tests that were failing due to setting System.out to null.

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/ReadFromStorage.java:19, tag dataflow_read_from_cloud_storage
• dataflow/snippets/src/main/java/com/example/dataflow/ReadFromStorage.java:44, tag dataflow_read_from_cloud_storage

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