Require an initialization call for "Dream.memory_sessions"

[boechat107]

Context

• We know that Dream.memory_sessions doesn't work properly when used with Dream.scope.
  • CSRF tokens not sessioned when using scope and memory_sessions #348
• There is an open PR trying to address the issue, but it has been inactive since November 10, 2024.
  • issue#348. Fixed CSRF tokens not sessioned when using scope and memory_sessions #349

Proposal

⚠️ API Breaking Changes!

This PR proposes adding one additional partial application for Dream.memory_sessions to make it possible to initialize its state before getting a handler.

(*==== Example 1 ====*)
let () =
  Dream.run
  @@ Dream.logger
  @@ Dream.memory_sessions ()
  @@ fun request -> ...

(*==== Example 2 ====*)
Dream.router [
  (* Note the partial application with unit. *)
  Dream.scope "/admin" [Dream.memory_sessions ()] [
    Dream.get "/" admin_handler;
    Dream.get "/logout" admin_logout_handler;
  ];
]

These changes should make it possible to share the session memory state with multiple routes inside a Dream.scope.

Notes

I tried first to change the function apply (router.ml) and make it generate a unified handler using router and building a route directly, but I realized this function wasn't designed for nested calls since it would return a 404 response when the request doesn't match a route.

[boechat107]

I believe the CI failures could be fixed by merging #383.