Add user_context to AmazonKendraRetriever (#8869)

### Description 

Now, we can pass information like a JWT token using user_context:  

```python
self.retriever = AmazonKendraRetriever(index_id=kendraIndexId, user_context={"Token": jwt_token})
```

- [x] `make lint`
- [x] `make format`
- [x] `make test`

Also tested by pip installing in my own project, and it allows access
through the token.

### Maintainers 

 @rlancemartin, @eyurtsev

### My twitter handle 

[girlknowstech](https://twitter.com/girlknowstech)

libs/langchain/langchain/retrievers/kendra.py

@@ -304,6 +304,9 @@ class AmazonKendraRetriever(BaseRetriever):
 
         client: boto3 client for Kendra
 
+        user_context: Provides information about the user context
+            See: https://docs.aws.amazon.com/kendra/latest/APIReference
+
     Example:
         .. code-block:: python
 
@@ -320,6 +323,7 @@ class AmazonKendraRetriever(BaseRetriever):
     attribute_filter: Optional[Dict] = None
     page_content_formatter: Callable[[ResultItem], str] = combined_text
     client: Any
+    user_context: Optional[Dict] = None
 
     @validator("top_k")
     def validate_top_k(cls, value: int) -> int:
@@ -368,6 +372,8 @@ class AmazonKendraRetriever(BaseRetriever):
         }
         if self.attribute_filter is not None:
             kendra_kwargs["AttributeFilter"] = self.attribute_filter
+        if self.user_context is not None:
+            kendra_kwargs["UserContext"] = self.user_context
 
         response = self.client.retrieve(**kendra_kwargs)
         r_result = RetrieveResult.parse_obj(response)