This allows working with a custom Langsmith base URL. For example,
I can use this to test against a local Langsmith instance, running on
my laptop with Docker by adding this to the proxy config:
```yaml
litellm_settings:
success_callback: ["langsmith"]
environment_variables:
LANGSMITH_BASE_URL: "http://localhost:1984"
LANGSMITH_PROJECT: "litellm-proxy"
```
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