C++ HTTP handlers - Module Bindings

[JasonAtClockwork]

Description of Changes

Adding C++ HTTP handlers based on #4636

• adds the C++ handler/router API SPACETIMEDB_HTTP_HANDLER(), SPACETIMEDB_HTTP_ROUTER()
• wires C++ HTTP handlers into module definition/build/runtime registration
• mirrors the Rust/TypeScript HTTP smoketests and docs coverage
• update to the the HTTP handlers docs examples

API and ABI breaking changes

New APIs for http handler and router behind SPACETIMEDB_UNSTABLE_FEATURES and should not be breaking.

Expected complexity level and risk

3 - this touches the C++ binding surface, runtime/module registration, and the smoketest/docs paths, so there is more churn than I’d like, but it is still fairly contained to the C++ HTTP work.

Testing

• Expanded the C++ compile-surface tests
• Added C++ unit coverage for HTTP conversion helpers
• Expanded crates/smoketests/tests/smoketests/http_routes.rs with C++ mirrors of the Rust HTTP
  route tests
• Added a C++ docs-example smoketest for docs/docs/00200-core-concepts/00200-functions/00600-HTTP- handlers.md

I also did some manual testing with a throw away project