feat: Add RAG LLMReranker

[ishaansehgal99]

Reason for Change:
Code for adding simple LLMReranker

This is common practice. Using the same LLM that is used for inference, to rerank RAG Results.

Sources:
https://docs.llamaindex.ai/en/stable/examples/workflow/rag/
https://www.llamaindex.ai/blog/using-llms-for-retrieval-and-reranking-23cf2d3a14b6
https://docs.llamaindex.ai/en/stable/module_guides/deploying/query_engine/usage_pattern/
https://docs.llamaindex.ai/en/stable/examples/node_postprocessor/LLMReranker-Gatsby/
https://medium.com/@abul.aala.fareh/different-reranking-techniques-in-llamaindex-6a56ed1f30a3
https://docs.llamaindex.ai/en/stable/examples/node_postprocessor/NVIDIARerank/

[zhuangqh]

low priority: we can reorg these params to cli args and using structured params instead of raw key-value dict.

[zhuangqh]

unit test fails

__________ ERROR collecting presets/ragengine/tests/api/test_main.py ___________
presets/ragengine/tests/api/test_main.py:8: in <module>
    from ragengine.main import app, vector_store_handler, rag_ops
presets/ragengine/main.py:5: in <module>
    from vector_store_manager.manager import VectorStoreManager
presets/ragengine/vector_store_manager/manager.py:7: in <module>
    from ragengine.vector_store.base import BaseVectorStore
presets/ragengine/vector_store/base.py:25: in <module>
    class BaseVectorStore(ABC):
presets/ragengine/vector_store/base.py:96: in BaseVectorStore
    rerank_params: Optional[Dict] = None
E   NameError: name 'Optional' is not defined
___ ERROR collecting presets/ragengine/tests/vector_store/test_base_store.py ___
presets/ragengine/tests/vector_store/test_base_store.py:9: in <module>
    from ragengine.vector_store.base import BaseVectorStore
presets/ragengine/vector_store/base.py:25: in <module>
    class BaseVectorStore(ABC):
presets/ragengine/vector_store/base.py:96: in BaseVectorStore
    rerank_params: Optional[Dict] = None
E   NameError: name 'Optional' is not defined
_ ERROR collecting presets/ragengine/tests/vector_store/test_chromadb_store.py _
presets/ragengine/tests/vector_store/test_chromadb_store.py:9: in <module>
    from ragengine.tests.vector_store.test_base_store import BaseVectorStoreTest
<frozen importlib._bootstrap>:1360: in _find_and_load
    ???
<frozen importlib._bootstrap>:1331: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:935: in _load_unlocked
    ???
../../../.local/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:184: in exec_module
    exec(co, module.__dict__)
presets/ragengine/tests/vector_store/test_base_store.py:9: in <module>
    from ragengine.vector_store.base import BaseVectorStore
presets/ragengine/vector_store/base.py:25: in <module>
    class BaseVectorStore(ABC):
presets/ragengine/vector_store/base.py:96: in BaseVectorStore
    rerank_params: Optional[Dict] = None
E   NameError: name 'Optional' is not defined
__ ERROR collecting presets/ragengine/tests/vector_store/test_faiss_store.py ___
presets/ragengine/tests/vector_store/test_faiss_store.py:8: in <module>
    from ragengine.tests.vector_store.test_base_store import BaseVectorStoreTest
<frozen importlib._bootstrap>:1360: in _find_and_load
    ???
<frozen importlib._bootstrap>:1331: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:935: in _load_unlocked
    ???
../../../.local/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:184: in exec_module
    exec(co, module.__dict__)
presets/ragengine/tests/vector_store/test_base_store.py:9: in <module>
    from ragengine.vector_store.base import BaseVectorStore
presets/ragengine/vector_store/base.py:25: in <module>
    class BaseVectorStore(ABC):
presets/ragengine/vector_store/base.py:96: in BaseVectorStore
    rerank_params: Optional[Dict] = None
E   NameError: name 'Optional' is not defined

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[Fei-Guo]

What are the list of parameters that we can change here?