Add global region to VertexAiRegion

[stephenleo]

The VertexAI global endpoint automatically routes the request to the most optimum location.

More info on the global region here: https://github.com/GoogleCloudPlatform/generative-ai/blob/main/gemini/global-endpoint/intro_global_endpoint.ipynb

[hyperlint-ai]

PR Change Summary

Added support for the 'global' region in VertexAI, allowing requests to be routed to the most optimal location.

• Introduced the 'global' region option for VertexAI requests.
• Updated documentation to clarify the use of the 'global' endpoint.
• Provided a link to model availability for better user guidance.

Modified Files

• docs/models.md

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