Support txtai as a vector store (#10240)

docs/community/integrations/vector_stores.md

docs/examples/vector_stores/TxtaiIndexDemo.ipynb

docs/module_guides/storing/vector_stores.md

llama_index/readers/__init__.py

llama_index/readers/txtai.py

llama_index/vector_stores/__init__.py

llama_index/vector_stores/registry.py

llama_index/vector_stores/txtai.py

tests/indices/vector_store/conftest.py

tests/indices/vector_store/mock_txtai.py

tests/indices/vector_store/test_txtai.py