Introduce Google Generative Language Semantic Retriever (#9440)

docs/community/integrations/managed_indices.md

docs/examples/managed/GoogleDemo.ipynb

docs/module_guides/querying/retriever/retrievers.md

llama_index/indices/managed/google/generativeai/__init__.py

llama_index/indices/managed/google/generativeai/base.py

...dex/response_synthesizers/google/generativeai/__init__.py

...a_index/response_synthesizers/google/generativeai/base.py

llama_index/vector_stores/google/generativeai/__init__.py

llama_index/vector_stores/google/generativeai/base.py

...ndex/vector_stores/google/generativeai/genai_extension.py

poetry.lock

pyproject.toml

tests/indices/managed/__init__.py

tests/indices/managed/test_google.py

tests/response_synthesizers/test_google.py

tests/vector_stores/test_google.py