Fixes token logging in callbacks when streaming=True is used. (#241)

Fixes #240
Fixes #217 

### Code to verify
```python
from langchain_aws import ChatBedrock
from langchain.callbacks.base import BaseCallbackHandler
from langchain_core.prompts import ChatPromptTemplate

streaming = True

class MyCustomHandler(BaseCallbackHandler):
    def on_llm_new_token(self, token: str, **kwargs) -> None:
        print(f"My custom handler, token: {token}")

prompt = ChatPromptTemplate.from_messages(["Tell me a joke about {animal} in a few words."])

model = ChatBedrock(
    model_id="anthropic.claude-3-haiku-20240307-v1:0",
    streaming = streaming, 
    callbacks=[MyCustomHandler()]
)

chain = prompt | model

response = chain.invoke({"animal": "bears"})
```

### Output
```
My custom handler, token: 
My custom handler, token: Bear
My custom handler, token: -
My custom handler, token: ly funny
My custom handler, token: .
My custom handler, token: 
My custom handler, token: 
```

libs/aws/langchain_aws/chat_models/bedrock.py

@@ -477,14 +477,19 @@ def _stream(
             **kwargs,
         ):
             if isinstance(chunk, AIMessageChunk):
-                yield ChatGenerationChunk(message=chunk)
+                generation_chunk = ChatGenerationChunk(message=chunk)
+                if run_manager:
+                    run_manager.on_llm_new_token(
+                        generation_chunk.text, chunk=generation_chunk
+                    )
+                yield generation_chunk
             else:
                 delta = chunk.text
                 if generation_info := chunk.generation_info:
                     usage_metadata = generation_info.pop("usage_metadata", None)
                 else:
                     usage_metadata = None
-                yield ChatGenerationChunk(
+                generation_chunk = ChatGenerationChunk(
                     message=AIMessageChunk(
                         content=delta,
                         response_metadata=chunk.generation_info,
@@ -493,6 +498,11 @@ def _stream(
                     if chunk.generation_info is not None
                     else AIMessageChunk(content=delta)
                 )
+                if run_manager:
+                    run_manager.on_llm_new_token(
+                        generation_chunk.text, chunk=generation_chunk
+                    )
+                yield generation_chunk
 
     def _generate(
         self,