restructure and testing different decision layer structures

semantic_router/encoders/__init__.py

semantic_router/layer.py

semantic_router/matchers/__init__.py

semantic_router/matchers/base.py

semantic_router/matchers/ranker_only.py

semantic_router/matchers/two_stage.py

semantic_router/rankers/__init__.py

semantic_router/rankers/base.py

semantic_router/rankers/cohere.py

semantic_router/retrievers/__init__.py

semantic_router/retrievers/base.py

semantic_router/retrievers/bm25.py

semantic_router/retrievers/cohere.py

semantic_router/retrievers/huggingface.py

semantic_router/retrievers/openai.py

semantic_router/schema.py