http: Make request writing functions public

Websocket client connection starts by a plain HTTP request with some
websocket-specific headers. Due to this similarity it makes sense to
reuse existing HTTP code when implementing websocket client in the
future. For that at least request_line() and write_request_headers() are
required.

These functions are accessible to existing HTTP client code due to
friend class experimental::connection being present.

include/seastar/http/request.hh

@@ -313,10 +313,10 @@ struct request {
      */
     static request make(httpd::operation_type type, sstring host, sstring path);
 
-private:
-    void add_query_param(std::string_view param);
     sstring request_line() const;
     future<> write_request_headers(output_stream<char>& out) const;
+private:
+    void add_query_param(std::string_view param);
     friend class experimental::connection;
 };