feat(vertexai): Add function calling basic

[Deleplace]

This sample was previously approved and merged, but later overwritten with a more advanced sample.

Now the 2 samples are needed in the documentation, so I copied and adapted the original go sample (basic) in a new file functioncalling_basic.go

The basic sample "getCurrentWeather" was ported from [the Python sample]

The new region tag is [START generativeaionvertexai_gemini_function_calling]

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• vertexai/function-calling/functioncalling_basic.go:24, tag generativeaionvertexai_gemini_function_calling

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