feat: Adding sample for pre-split feature

[aksharauke]

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Fixes #issue_number_goes_here 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/database-add-split-points.js:28, tag spanner_database_add_split_points

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[generated-files-bot]

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

• 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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