Add DocumentTransformer abstraction so that in #2915 we don't have to
wrap TextSplitter and RedundantEmbeddingFilter (neither of which uses
the query) in the contextual doc compression abstractions. with this
change, doc filter (doc extractor, whatever we call it) would look
something like
```python
class BaseDocumentFilter(BaseDocumentTransformer[_RetrievedDocument], ABC):
@abstractmethod
def filter(self, documents: List[_RetrievedDocument], query: str) -> List[_RetrievedDocument]:
...
def transform_documents(self, documents: List[_RetrievedDocument], query: Optional[str] = None, **kwargs: Any) -> List[_RetrievedDocument]:
if query is None:
raise ValueError("Must pass in non-null query to DocumentFilter")
return self.filter(documents, query)
```
10e4b32ecb