Add UserAgent header "llamaindex-python" for azure search (#16895)

.../docs/examples/vector_stores/AzureAISearchIndexDemo.ipynb

...eaisearch/llama_index/vector_stores/azureaisearch/base.py

...es/llama-index-vector-stores-azureaisearch/pyproject.toml

...x-vector-stores-azureaisearch/tests/test_azureaisearch.py