Add Server::CustomRoute() for HTTP methods outside the built-in set (#2553)

* Add Server::CustomRoute() for HTTP methods outside the built-in set

parse_request_line validates the request method against a fixed whitelist and
rejects anything else with 400 before routing runs. That blocks WebDAV, where
PROPFIND, PROPPATCH, MKCOL, COPY, MOVE, LOCK and UNLOCK are ordinary methods
defined by RFC 4918, and it blocks extension methods such as UPnP's SUBSCRIBE.
The need has been open since #847.

Registering a handler is now what makes the server accept a method:

    svr.CustomRoute("PROPFIND", "/dav/:id", handler);

Because custom methods go through the normal dispatch path, patterns work the
way they do for Get() and friends, and the request body is available in
req.body. Serving these methods through set_pre_routing_handler was never
enough: the body has not been read at that point, so PROPPATCH and LOCK, which
require one, could not be implemented at all.

A HandlerWithContentReader overload is available too. The content reader gate
in routing() also fires when a custom method carries no body, matching what
expect_content() does unconditionally for POST/PUT/PATCH/DELETE, so a body-less
PROPFIND (RFC 4918 treats one as allprop) reaches its handler instead of
falling through to 404.

Method names are validated as RFC 9110 tokens, and the ten built-in methods are
refused. Seven of them are dispatched by the if/else chain in routing() before
the custom tables are consulted, so a route registered for one could never
fire; CONNECT, TRACE and PRI carry protocol-level meaning this library does not
route. A refused registration makes is_valid() return false, so listen() fails
rather than starting a server holding a handler that would never run. This is
also why SSLServer::is_valid() now chains to Server::is_valid() instead of only
checking ctx_.

Servers that never call CustomRoute() keep the previous per-request cost: the
built-in method set is checked first and short-circuits, and the custom lookup
returns early on an empty map.

* Add cookbook recipe for custom HTTP methods

The CustomRoute() docs were a section inside S01, which pushed that page to 90
lines, the longest in the cookbook, and mixed a separate feature into a page
about registering GET/POST/PUT/DELETE handlers. Move the section into its own
recipe and give it room for the part that was missing: the OPTIONS handler
returning DAV: and Allow, which WebDAV clients probe for before anything else.
S01 goes back to 68 lines and keeps a pointer to the new page.

The recipe is titled after the API rather than after WebDAV, and says outright
that generating the 207 Multi-Status XML, interpreting Depth and managing locks
are the reader's job. Routing the method is all the library does.

S23 takes order 42, so the TLS, SSE and WebSocket recipes shift to 43-57. That
only moves the sort key. Filenames, the T01/E01/W01 labels, the published URLs
and every cross-reference are untouched.

README.md

@@ -307,6 +307,39 @@ int main(void)
 
 `Post`, `Put`, `Patch`, `Delete` and `Options` methods are also supported.
 
+### Custom HTTP methods
+
+Methods outside the built-in set are rejected with `400 Bad Request` unless a handler is registered for them with `CustomRoute`. This covers the WebDAV methods of RFC 4918, `SUBSCRIBE` and friends from UPnP, and any other extension method.
+
+```c++
+svr.CustomRoute("PROPFIND", "/dav/:id", [](const Request& req, Response& res) {
+  // The request body is available as usual
+  auto id = req.path_params.at("id");
+  res.status = StatusCode::MultiStatus_207;
+  res.set_content(build_multistatus(req.body), "application/xml");
+});
+
+// A content reader overload is available too
+svr.CustomRoute("REPORT", "/dav/.*",
+                [](const Request& req, Response& res,
+                   const ContentReader& content_reader) {
+                  content_reader([&](const char* data, size_t data_length) {
+                    // ...
+                    return true;
+                  });
+                });
+```
+
+Patterns work exactly as they do for `Get` and the other methods, so regular expressions and path parameters are both available.
+
+Note the following:
+
+* The method name must be a valid HTTP method token (RFC 9110) and must be registered before `listen()` is called.
+* `GET`, `HEAD`, `POST`, `PUT`, `DELETE`, `CONNECT`, `OPTIONS`, `TRACE`, `PATCH` and `PRI` cannot be registered this way. Use the dedicated methods above instead.
+* A rejected registration makes `is_valid()` return `false`, and `listen()` then fails rather than starting a server with a route that would never fire.
+* Static file serving and WebSocket upgrades remain `GET`/`HEAD` only.
+* `Allow` and the WebDAV `DAV:` header are not generated automatically. Register an `Options` handler if clients need them.
+
 ### Bind a socket to multiple interfaces and any available port
 
 ```cpp