feat(attachments): add json attachment uploads

[Abhijeet Prasad (AbhiPrasad)]

Add a public JSON attachment API that serializes JSON, stores an attachment reference on an OpenTelemetry span, and uploads the JSON payload in the background. The upload queue is owned by the Braintrust client and is flushed or shut down with the configured tracer provider/span processor.

New public API:

Initialize Braintrust with a tracer provider and the normal client options:

tp := sdktrace.NewTracerProvider()
defer tp.Shutdown(context.Background())

bt, err := braintrust.New(tp,
    braintrust.WithAPIKey(os.Getenv("BRAINTRUST_API_KEY")),
    braintrust.WithProject("my-project"),
)
if err != nil {
    log.Fatal(err)
}

tracer := bt.Tracer("my-app")

Send a JSON attachment by starting a span and passing that span to the client:

ctx, span := tracer.Start(context.Background(), "example")
defer span.End()

_, err = bt.SetJSONAttachment(ctx, span, "braintrust.input_json", map[string]any{
    "messages": []map[string]string{
        {"role": "user", "content": "Summarize this conversation."},
    },
},
    attachment.WithFilename("conversation.json"),
    attachment.WithPrettyJSON(),
)
if err != nil {
    log.Fatal(err)
}