Track costs for streaming with Cohere

[brenkao]

Is your feature request related to a problem? Please describe.
Many providers are starting to add usage to streaming. This makes it much easier for Mirascope to calculate cost.

Describe the solution you'd like
Add a total_cost property to CohereCallResponseChunk. Read the "event_type": "stream-end" sent by Cohere API and calculate cost using

"token_count": {
    "prompt_tokens": ...,
    "response_tokens": ...,
    "total_tokens": ...,
    "billed_tokens": ...,
}

Update https://github.com/Mirascope/mirascope/blob/dev/mirascope/cohere/utils.py as necessary.

[willbakst]

See #214 since these are related.

Namely: #214 (comment)