Pull request: websvc: add system info

Merge in DNS/adguard-home from websvc-system-info to master

Squashed commit of the following:

commit 333aaa0602da254e25e0262a10080bf44a3718a7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 12 16:32:32 2022 +0300

    websvc: fmt

commit d8a35bf71dcc59fdd595494e5b220e3d24516728
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 12 16:10:11 2022 +0300

    websvc: refactor, imp tests

commit dfeb24f3f35513bf51323d3ab6f717f582a1defc
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 11 20:52:02 2022 +0300

    websvc: add system info

internal/v1/websvc/middleware.go

@@ -0,0 +1,16 @@
+package websvc
+
+import "net/http"
+
+// Middlewares
+
+// jsonMw sets the content type of the response to application/json.
+func jsonMw(h http.Handler) (wrapped http.HandlerFunc) {
+	f := func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+
+		h.ServeHTTP(w, r)
+	}
+
+	return http.HandlerFunc(f)
+}