Allow more authentication types

[tarunwadhwa13]

Is your feature request related to a problem? Please describe.
Currently, Agenta mostly supports API Keys as authentication type for models which works for most vendors. However for self hosted models connecting to GCP (or in some cases even for AWS), there might be a requirement to provide info more than just the API Key (AWS for example requires specifying region name and GCP requires service json file)

Describe the solution you'd like
LiteLLM already supports using AWS specific env variables or GCP service account file.
I understand there might be a lot of complexities in current app design but this could give access to more enterprise models in a secure way

Enhancing the current LLM key design to choosing a dropdown for auth mechanism which could be passed to litellm. (Adds some vendor specific code though 😢)

Describe alternatives you've considered
No hacks yet. For AWS, you are bound to host on AWS ec2 only