feat: Add Vertex AI provider for Google Cloud Gemini access (#28) #29

Job	Run time
Compile & Warnings	23s
Tests (ubuntu-latest / OTP 27.2 / Elixir 1.18.3)	30s
Credo	21s
Formatting	14s
Dialyzer	2m 34s
Publish to Hex	1m 8s
	5m 10s

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