samples: add samples for object retention

[ddelgrosso1]

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Fixes #2408 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/createBucketWithObjectRetention.js:23, tag storage_create_bucket_with_object_retention
• samples/setObjectRetentionPolicy.js:27, tag storage_set_object_retention_policy

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[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml
• samples/README.md - samples/README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml