Set up initial Vertex AI framework

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Add the basic build logic for the Vertex AI SDK. This includes the CMake file, and the scripts for building and packaging. Note: this doesn't update the GHA workflow defaults, so it will not be included in builds unless explicitly added.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/12916702916

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Type of Change

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• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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