feat: Add listing llm models and embedding models for Azure endpoint

[mattzh72]

Description

Add listing llm models and embedding models for Azure endpoint. We also make some changes to the api_version field for Azure:

• we remove it from LLMConfig as it is a field only necessary for the Azure endpoint
• we make it an optional field for the model settings
• if the env variable is missing, we default to the latest api version (hardcoded)

Testing

Listed out LLMs:

[LLMConfig(model='gpt-35-turbo-0301', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-35-turbo-0301/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048), LLMConfig(model='gpt-35-turbo-0613', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-35-turbo-0613/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048), LLMConfig(model='gpt-35-turbo-1106', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-35-turbo-1106/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048), LLMConfig(model='gpt-35-turbo-0125', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-35-turbo-0125/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048), LLMConfig(model='gpt-35-turbo-16k-0613', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-35-turbo-16k-0613/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048), LLMConfig(model='gpt-4-0125-Preview', model_endpoint_type='azure', model_endpoint='https://letta.openai.azure.com/openai/deployments/gpt-4-0125-Preview/chat/completions?api-version=2024-09-01-preview', model_wrapper=None, context_window=2048)...

Listed out embeddings:

tests/test_providers.py::test_azure_openai [EmbeddingConfig(embedding_endpoint_type='azure', embedding_endpoint='https://letta.openai.azure.com/openai/deployments/text-embedding-ada-002/embeddings?api-version=2024-09-01-preview', embedding_model='text-embedding-ada-002', embedding_dim=768, embedding_chunk_size=300, azure_endpoint=None, azure_version=None, azure_deployment=None)...

[sarahwooders]

lgtm!