samples: samples for postgreSQL

[asthamohta]

currently build is fling but should be okay with this PR merged: #1610

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/pg-add-column.js:26, tag spanner_postgresql_add_column
• samples/pg-dml-getting-started-update.js:26, tag spanner_postgresql_dml_getting_started_update
• samples/pg-index-create-storing.js:26, tag spanner_postgresql_create_storing_index

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[generated-files-bot]

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