Move models param to the endpoint rather than constructor

[billytrend-cohere]

This PR removes the optional chat_model, embed_model, and generate_model parameters from the __init__ method of the AwsClient class and its subclasses BedrockClient and SagemakerClient. It also updates the test cases in tests/test_aws_client.py to use the models dictionary instead of hardcoding the model values.

Summary of Changes

• Removes the optional chat_model, embed_model, and generate_model parameters from the __init__ method of the AwsClient class and its subclasses.
• Updates the test cases in tests/test_aws_client.py to use the models dictionary for model values instead of hardcoding them.
• Renames the client variable to models in the TestClient class.
• Adds the models parameter to the parameterized_class decorator in tests/test_aws_client.py.