feat(aiplatform): add initial Imagen code sample and test

[irataxy]

Description

Fixes #355484599

• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

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• aiplatform/src/main/java/aiplatform/imagen/GenerateImageSample.java:19, tag generativeaionvertexai_imagen_generate_image

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[irataxy]

@minherz I'm not sure the tests are running. I don't see individual test cases in the log.

[msampathkumar]

LGTM

[msampathkumar]

Update: Code sample look good to me. I hoping for a second reviewer to check the repo's specific code style(if any) & merge the PR.

[Sita04]

Few nits. Kindly address those before merging. Thanks!