added run_async method

[The-EDev]

also added unit tests for #358

Closes #351

[The-EDev]

CI is failing because it uses code from #358.

CI should be restarted once #358 is merged.

[crow-clang-format]

--- include/crow/app.h	(before formatting)
+++ include/crow/app.h	(after formatting)
@@ -307,7 +307,9 @@
         /// Non-blocking version of \ref run()
         std::future<void> run_async()
         {
-            return std::async(std::launch::async, [&] {this->run();});
+            return std::async(std::launch::async, [&] {
+                this->run();
+            });
         }
 
         /// Stop the server
--- tests/unittest.cpp	(before formatting)
+++ tests/unittest.cpp	(after formatting)
@@ -532,7 +532,7 @@
         return "B";
     });
 
-    auto _  = app1.bindaddr(LOCALHOST_ADDRESS).port(45451).run_async();
+    auto _ = app1.bindaddr(LOCALHOST_ADDRESS).port(45451).run_async();
     auto _2 = app2.bindaddr(LOCALHOST_ADDRESS).port(45452).run_async();
     app1.wait_for_server_start();
     app2.wait_for_server_start();