feat: add Gemini batch prediction samples for GCS and BQ

[irataxy]

Description

Fixes b:373871285

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Checklist

• [X] I have followed Sample Format Guide

• [X] Tests pass: mvn clean verify required

• [X] Lint passes: mvn -P lint checkstyle:check required

• [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.

• aiplatform/src/main/java/aiplatform/batchpredict/CreateBatchPredictionGeminiBigqueryJobSample.java:19, tag generativeaionvertexai_batch_predict_gemini_createjob_bigquery
• aiplatform/src/main/java/aiplatform/batchpredict/CreateBatchPredictionGeminiJobSample.java:19, tag generativeaionvertexai_batch_predict_gemini_createjob_gcs

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