feat: add support for UpdateDatabase

[rajatbhatta]

This PR adds code changes, unit tests and integration tests for UpdateDatabase support in Cloud Spanner.

[snippet-bot]

Here is the summary of changes.

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• samples/database-update.js:26, tag spanner_update_database

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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

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