Fix CohereRerank configuration (langchain-ai#11583)

**Description:** CohereRerank is missing `cohere_api_key` as a field and
since extras are forbidden, it is not possible to pass-in the key. The
only way is to use an env variable named `COHERE_API_KEY`.

For example, if trying to create a compressor like this:
```python
cohere_api_key = "......Cohere api key......"
compressor = CohereRerank(cohere_api_key=cohere_api_key)
```
you will get the following error:
```
  File "/langchain/.venv/lib/python3.10/site-packages/pydantic/v1/main.py", line 341, in __init__
    raise validation_error
pydantic.v1.error_wrappers.ValidationError: 1 validation error for CohereRerank
cohere_api_key
  extra fields not permitted (type=value_error.extra)
```

libs/langchain/langchain/retrievers/document_compressors/cohere_rerank.py

@@ -29,6 +29,8 @@ class CohereRerank(BaseDocumentCompressor):
     model: str = "rerank-english-v2.0"
     """Model to use for reranking."""
 
+    cohere_api_key: Optional[str] = None
+
     class Config:
         """Configuration for this pydantic object."""