Add JSON profiling support for HTTP stats

[tobilg]

This PR adds structured JSON profiling output for HTTPFS statistics, complementing the existing text-based profiling.

This is in preparation of another PR on DuckDB itself that will add comprehensive NDJSON logging and profiling support eventually. I will link the DuckDB PR once it's ready @carlopi. Thanks!

Changes

• Added WriteProfilingInformationToJSON() method to HTTPState class
• Exports HTTP metrics as structured JSON: total_bytes_received, total_bytes_sent, head_count, get_count, put_count, post_count, delete_count
• Uses DuckDB's yyjson library for JSON construction

Forward Compatibility

The method is implemented without override for forward compatibility. When DuckDB adds WriteProfilingInformationToJSON() to ClientContextState, this implementation will automatically override it. Adding override at that time is recommended for compile-time safety.