Adding terraform to setup spanner IT infra

[sm745052]

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[gemini-code-assist]

Summary of Changes

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This pull request introduces documentation and setup guidance for managing Spanner integration test infrastructure. By providing Terraform-based instructions, it simplifies the environment setup process for vendors and PSOs, ensuring consistent and reproducible test environments.

Highlights

• Documentation Update: Added instructions to the contributor documentation for setting up Spanner test infrastructure using Terraform.
• New Infrastructure Guide: Created a new README file in the test-infra/terraform/spanner directory to guide users through provisioning and cleaning up Spanner resources.

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