[C++] Add Lundi framework

[olafurjohannsson]

Add Lundi C++20 framework

Lundi is a header-only, asynchronous C++20 web framework using Asio coroutines.

Test types implemented

All 7: json, plaintext, db, queries, fortunes, updates, cached-worlds

Implementation details

• Async non-blocking libpq with pipeline mode (libpq 14+), fallback to sequential for older versions
• Per-thread connection pool using asio::experimental::channel
• Zero-allocation JSON writer using write_buffer + fast_itoa
• Prepared statements for all queries

Verification

All 7 tests pass locally with ./tfb --mode verify --test lundi