feat(batch): add Batch create custom network sample

[TetyanaYahodska]

Description

Fixes #

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Checklist

• [x] 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)
• [x] Tests pass: mvn clean verify required
• [x] 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

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [batch/snippets/src/main/java/com/example/batch/CreateBatchCustomNetwork.java:17](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/a6aa0ffc5897f574548238db16ec1a5c6761aaa3/batch/snippets/src/main/java/com/example/batch/CreateBatchCustomNetwork.java#L17), tag `batch_create_custom_network`

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