feat: support for different MetadataStores in VectorStore

[akonarski-ds]

VectorStore class should support different ways of storing metadata that is associated with stored vectors. Some of the vector stores providers allow to store metadata in the same DB as vectors, but some of them are missing such support.
To solve this we can add field to VectorStore class that will allow to provide external MetadataStore:

class MetadataStore(ABC):
     ...
     async def store(self, key: str | UUID, metadata: dict) -> None:
          ...

     async def query(self, query: str) -> dict:
         ...

     async def get(self, key: str | UUID) -> dict:
        ...


class VectorStore:
    metadata_store: MetadataStore
    ...

    def __init__(self, metadata_store: MetadataStore = DEFAULT_METADATA_STORE):