feat: Managed Autoscaler

[alkatrivedi]

This PR contains sample and its integration test for the managed autoscaler feature in NodeJs Client Library.
Sample: instance-with-autoscaling-config.js

[snippet-bot]

Here is the summary of changes.

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• samples/instance-with-autoscaling-config.js:24, tag spanner_create_instance_with_autoscaling_config

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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