Add Amazon Bedrock knowledge base integration as retriever (#12737)

docs/docs/examples/retrievers/bedrock_retriever.ipynb

...ions/retrievers/llama-index-retrievers-bedrock/.gitignore

...egrations/retrievers/llama-index-retrievers-bedrock/BUILD

...ations/retrievers/llama-index-retrievers-bedrock/Makefile

...tions/retrievers/llama-index-retrievers-bedrock/README.md

...ons/retrievers/llama-index-retrievers-bedrock/__init__.py

...x-retrievers-bedrock/llama_index/retrievers/bedrock/BUILD

...ievers-bedrock/llama_index/retrievers/bedrock/__init__.py

...retrievers-bedrock/llama_index/retrievers/bedrock/base.py

.../retrievers/llama-index-retrievers-bedrock/pyproject.toml

...ons/retrievers/llama-index-retrievers-bedrock/tests/BUILD

...trievers/llama-index-retrievers-bedrock/tests/__init__.py

...index-retrievers-bedrock/tests/test_retrievers_bedrock.py