feat: add tpu_vm_create_topology/startup_script

[gryczj]

Description

Fixes #

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Checklist

• [x] I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
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• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
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• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

- [tpu/createStartupScriptVM.js:20](https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/a5d0ca1e438777d5d660f68c02a1ab0f213d4b54/tpu/createStartupScriptVM.js#L20), tag `tpu_vm_create_startup_script` - [tpu/createTopologyVM.js:20](https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/a5d0ca1e438777d5d660f68c02a1ab0f213d4b54/tpu/createTopologyVM.js#L20), tag `tpu_vm_create_topology`

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[gryczj]

Due to high costs of TPU Nodes, it was decided to use mocks in the tests.