Add VectorStore integration for Vespa (#13213)

docs/docs/examples/vector_stores/VespaIndexDemo.ipynb

.../vector_stores/llama-index-vector-stores-vespa/.gitignore

...tions/vector_stores/llama-index-vector-stores-vespa/BUILD

...ector_stores/llama-index-vector-stores-vespa/CHANGELOG.md

...ns/vector_stores/llama-index-vector-stores-vespa/Makefile

...s/vector_stores/llama-index-vector-stores-vespa/README.md

...vector-stores-vespa/llama_index/vector_stores/vespa/BUILD

...-stores-vespa/llama_index/vector_stores/vespa/__init__.py

...ctor-stores-vespa/llama_index/vector_stores/vespa/base.py

...stores-vespa/llama_index/vector_stores/vespa/templates.py

...tor_stores/llama-index-vector-stores-vespa/pyproject.toml

...vector_stores/llama-index-vector-stores-vespa/tests/BUILD

..._stores/llama-index-vector-stores-vespa/tests/__init__.py

...-index-vector-stores-vespa/tests/test_vespavectorstore.py