Adding Compatibility for Cohere Command-R model family API

[keshavd]

Cohere's Command-R and Command-R-Plus models have a different API than the Command model provided on Bedrock.

This pull request adds compatibility based on model ID instead of the provider exclusively.

closes https://github.com/langchain-ai/langchain-aws/issues/66

[3coins]

@keshavd I am not able to test the cohere model with these changes. Here is the sample code I used.

params = {
    "region_name": "us-west-2",
    "model_id": "cohere.command-r-v1:0"
}
model = BedrockLLM(**params)
response = model.invoke("Hello!")

Here is the exception thrown in _prepare_input_and_invoke

try:
      response = self.client.invoke_model(**request_options)

      (
          text,
          tool_calls,
          body,
          usage_info,
          stop_reason,
      ) = LLMInputOutputAdapter.prepare_output(provider, response).values()

  except Exception as e:
>           raise ValueError(f"Error raised by bedrock service: {e}")
E           ValueError: Error raised by bedrock service: 'NoneType' object is not subscriptable