Implement new store and metadata params

[akshaylb]

Is your feature request related to a problem? Please describe.
OpenAI recently introduced new parameters for distillation.

Describe the solution you'd like
Implement store and metadata fields for the chat completion request. And add a .WithStore(bool) and .WithMetadata(map[string]string) for the OpenAI Client.

[unitythemaker]

I think this is added with: #870

Not sure about .WithStore(bool) and .WithMetadata(map[string]string) though.

[unitythemaker]

Hmm, it seems like Store option is not added yet for Chat Completions.

[rmarku]

metadata is object, so it should allow things like this:

 "metadata": {
        "tags": ["testTag", "taskName:pass0", "company:meitar"]
    }

This is in particular used by LiteLLM Proxy.