feat: Upwards-recursively read .env.braintrust containing BRAINTRUST_API_KEY on login in Node.js

[Luca Forstner (lforst)]

We are planning on a change where the wizard will generate a .env.braintrust file with a BRAINTRUST_API_KEY into the cwd.

This is so the user can use the wizard and immediately verify the generated instrumentation locally.

Some thoughts around this implementation:

• Should be lazy and non-blocking
• The implementation walks the directories upwards when looking for .env.braintrust
• The walking upwards is kicked off in parallel for each folder, however the closest one wins
• In other runtimes than Node.js this does change does not make too much sense
• For sending otel spans, the exporter blocks until we found a matching file