samples: add support for updateDatabase in Cloud Spanner

[aayushimalik]

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[snippet-bot]

Here is the summary of changes.

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• samples/snippets/src/main/java/com/example/spanner/UpdateDatabaseSample.java:19, tag spanner_update_database

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[rajatbhatta]

@aayushimalik: Could you please use the sample shared internally in the client spec doc to write this sample? It is to maintain consistency in the sample across clients.

[rajatbhatta]

LGTM, apart from a few minor nits.