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/json.go

@@ -0,0 +1,61 @@
+package websvc
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// JSON Utilities
+
+// jsonTime is a time.Time that can be decoded from JSON and encoded into JSON
+// according to our API conventions.
+type jsonTime time.Time
+
+// type check
+var _ json.Marshaler = jsonTime{}
+
+// nsecPerMsec is the number of nanoseconds in a millisecond.
+const nsecPerMsec = float64(time.Millisecond / time.Nanosecond)
+
+// MarshalJSON implements the json.Marshaler interface for jsonTime.  err is
+// always nil.
+func (t jsonTime) MarshalJSON() (b []byte, err error) {
+	msec := float64(time.Time(t).UnixNano()) / nsecPerMsec
+	b = strconv.AppendFloat(nil, msec, 'f', 3, 64)
+
+	return b, nil
+}
+
+// type check
+var _ json.Unmarshaler = (*jsonTime)(nil)
+
+// UnmarshalJSON implements the json.Marshaler interface for *jsonTime.
+func (t *jsonTime) UnmarshalJSON(b []byte) (err error) {
+	if t == nil {
+		return fmt.Errorf("json time is nil")
+	}
+
+	msec, err := strconv.ParseFloat(string(b), 64)
+	if err != nil {
+		return fmt.Errorf("parsing json time: %w", err)
+	}
+
+	*t = jsonTime(time.Unix(0, int64(msec*nsecPerMsec)).UTC())
+
+	return nil
+}
+
+// writeJSONResponse encodes v into w and logs any errors it encounters.  r is
+// used to get additional information from the request.
+func writeJSONResponse(w io.Writer, r *http.Request, v interface{}) {
+	err := json.NewEncoder(w).Encode(v)
+	if err != nil {
+		log.Error("websvc: writing resp to %s %s: %s", r.Method, r.URL.Path, err)
+	}
+}