Improve Middleware documentation and compile time static assertions

[Guiorgy]

Closes #861

Currently there is some ambiguity in the documentation surrounding middlewares:

• CROW_MIDDLEWARES: In the reference docs the example uses crow::SimpleApp, yet the example in Crow/examples states // ALL middleware (including per handler) is listed and the guide doesn't mention this at all:

  auto app = [crow::SimpleApp](https://crowcpp.org/master/reference/namespacecrow.html#a3603179c9794548cac2c9990685178b4)(); // or crow::App()
  [CROW_ROUTE](https://crowcpp.org/master/reference/app_8h.html#a6aa7ec3a2f3ee3f17d5f9acd879e7381)(app, "/with_middleware")
  .CROW_MIDDLEWARES(app, LocalMiddleware) // Can be used more than one
  ([]() {                                 // middleware.
      return "Hello world!";
  });

  // ALL middleware (including per handler) is listed
  crow::App<RequestLogger, SecretContentGuard, RequestAppend> app;
  ...
  CROW_ROUTE(app, "/secret")
      // Enable SecretContentGuard for this handler
      .CROW_MIDDLEWARES(app, SecretContentGuard)([]() {
          return "";
      });

  It needs to be clarified, does CROW_MIDDLEWARES require the specified middleware be also specified in the referenced app template arguments?

• The reference docs mention the Crow constructor Construct Crow with a subset of middleware., yet this is not explained in the Guide, and there's no example of the usage in the examples.

  The guide should include a section on configuring middlewares, which should include both with the constructor and with app.get_middleware<>.

  The examples should include an example on using the app constructor.

Also, we can reduce the confusion by implementing compile time static assertions:

• When passing a middleware to the app constructor, it should be validated that the type of that middleware is within the middlewares defined in the app class template types.
• When passing a middleware to CROW_MIDDLEWARES, it should be validated that the type of that middleware is within the middlewares defined in the app class template types.

TODO:

• Guide: Local middleware need to be defined in the app middlewares.
• Reference docs: CROW_MIDDLEWARES comment and example.
• Guide: Middlewares configuration using app constructor and using app.get_middleware<>.
• Ecamples: Middleware configuration using app constructor.
• Validate middlewares passed to the app constructor.
• Validate middlewares passed to CROW_MIDDLEWARES.

As of writing, the PR includes an implementation that validates middlewares passed to the app constructor, however, the current implementation uses std::decay when comparing types. This needs to be discussed. It also needs more testing for edge cases IMO. Also, I'd like to do the same for CROW_MIDDLEWARES but I don't know where to do so, a suggestion would be appreciated.

[gittiver]

closed, author will reopen it when he will proceed to work on it.