feat(spanner): add samples for instance partitions

[varuncnaik]

Description

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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
• [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 1 region tag.

- [spanner/spanner_snippets/spanner/spanner_create_instance_partition.go:17](https://github.com/varuncnaik/golang-samples/blob/55485f426597b8f7e70e8de193f156930ddd7438/spanner/spanner_snippets/spanner/spanner_create_instance_partition.go#L17), tag `spanner_create_instance_partition`

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