feat(storage): add object retention samples

[BrennaEpp]

Description

Fixes #3803

Checklist

• [x] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [x] Tests pass: go test -v ./.. (see Testing)
• [x] Code formatted: gofmt (see Formatting)
• [x] Vetting pass: go vet (see Formatting)
• [ ] 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)
• [ ] 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 2 region tags.

• storage/buckets/create_bucket_object_retention.go:17, tag storage_create_bucket_with_object_retention
• storage/objects/set_object_retention.go:17, tag storage_set_object_retention_policy

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[muncus]

Apologies for the struggle with automerge. i've examined the failing tests, and they do not appear related to this change.