feat(vertexai): system instruction

[Deleplace]

Go version of the Python sample at https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/generative_ai/gemini_system_instruction.py

Region tag generativeaionvertexai_gemini_system_instruction

Note: using cloud.google.com/go/vertexai v0.7.2-0.20240430000329-84e3236355de as there is no tag yet that includes model.SystemInstruction

Edit: now using cloud.google.com/go/vertexai v0.8.0 — thank you jba

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• vertexai/system-instruction/systeminstruction.go:21, tag generativeaionvertexai_gemini_system_instruction

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[grayside]

Commenting with pending question on model.GenerateContent.