Add Usage.total_cost_usd: Total cost in USD (only in result message)

[appcollscott]

The Usage object now contains a field 'total_cost_usd' in the result message. Please add?

-- https://platform.claude.com/docs/en/agent-sdk/typescript#sdk-result-message

[appcollscott]

Thank you!

[tghamm]

Np! take a look at the readme for this one, there is no 1:1 proxy, but I think what I added should work for most cases as well as make it extensible if you are on some kind of special deal.