feat: Add Google Vertex AI inference provider support (#2841)

# What does this PR do?
- Add new Vertex AI remote inference provider with litellm integration
- Support for Gemini models through Google Cloud Vertex AI platform
- Uses Google Cloud Application Default Credentials (ADC) for
authentication
- Added VertexAI models: gemini-2.5-flash, gemini-2.5-pro,
gemini-2.0-flash.
- Updated provider registry to include vertexai provider
- Updated starter template to support Vertex AI configuration
- Added comprehensive documentation and sample configuration

<!-- If resolving an issue, uncomment and update the line below -->
relates to https://github.com/meta-llama/llama-stack/issues/2747

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

Signed-off-by: Eran Cohen <eranco@redhat.com>
Co-authored-by: Francisco Arceo <arceofrancisco@gmail.com>

tests/integration/inference/test_openai_completion.py

@@ -34,6 +34,7 @@ def skip_if_model_doesnt_support_openai_completion(client_with_models, model_id)
         "remote::runpod",
         "remote::sambanova",
         "remote::tgi",
+        "remote::vertexai",
     ):
         pytest.skip(f"Model {model_id} hosted by {provider.provider_type} doesn't support OpenAI completions.")
 

tests/integration/inference/test_text_inference.py

@@ -29,6 +29,7 @@ def skip_if_model_doesnt_support_completion(client_with_models, model_id):
             "remote::openai",
             "remote::anthropic",
             "remote::gemini",
+            "remote::vertexai",
             "remote::groq",
             "remote::sambanova",
         )