Return structured data instead of JSON string for extension fields #4

Summary

We suggest improving how the library returns data intended for insertion into extension fields. Currently, the library returns a JSON blob as a string, which introduces unnecessary overhead for consumers.

Problem

At the moment, extension data is returned as a JSON string. To access specific fields (e.g., treatment group for Amazon reporting), consumers must:

1. Unmarshal the JSON string into a struct/map
2. Extract the required fields

This adds extra CPU overhead and allocations, especially in high-throughput systems where this operation happens frequently.

Impact

• Increased marshaling/unmarshaling overhead
• latency in hot paths
• Higher CPU and memory usage under load

Expected behavior

The library should return structured data directly, allowing consumers to access fields without additional decoding.

Suggestion

Instead of returning a JSON string, consider returning:

• a map[string]interface{}
  or
• a well-defined struct representing the extension schema