Migrate braintrust-langchain into main braintrust package. #27
Description
Similar to what we did with the braintrust_adk package (https://github.com/braintrustdata/braintrust-sdk-python/tree/main/integrations/adk-py), let's deprecate braintrust-langchain and make it an integration in the main braintrust package. We can also make this autoenabled via python auto instrumentation.
See #13 for how we did this for the braintrust adk integration.
Previous attempt PR: braintrustdata/braintrust-sdk-javascript#1320