Pull request 1897: nextapi-write-conf

Squashed commit of the following:

commit 72f25ffe73d6b8216b01e590fba66fb5f6944113
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jun 28 21:29:04 2023 +0300

    next: add conf writing, validation

internal/next/configmgr/error.go

@@ -0,0 +1,27 @@
+package configmgr
+
+import (
+	"fmt"
+
+	"github.com/AdguardTeam/golibs/timeutil"
+	"golang.org/x/exp/constraints"
+)
+
+// numberOrDuration is the constraint for integer types along with
+// timeutil.Duration.
+type numberOrDuration interface {
+	constraints.Integer | timeutil.Duration
+}
+
+// newMustBePositiveError returns an error about the value that must be positive
+// but isn't.  prop is the name of the property to mention in the error message.
+//
+// TODO(a.garipov): Consider moving such helpers to golibs and use in AdGuardDNS
+// as well.
+func newMustBePositiveError[T numberOrDuration](prop string, v T) (err error) {
+	if s, ok := any(v).(fmt.Stringer); ok {
+		return fmt.Errorf("%s must be positive, got %s", prop, s)
+	}
+
+	return fmt.Errorf("%s must be positive, got %d", prop, v)
+}