p2p/dnsdisc: add implementation of EIP-1459 (#20094)

This adds an implementation of node discovery via DNS TXT records to the
go-ethereum library. The implementation doesn't match EIP-1459 exactly,
the main difference being that this implementation uses separate merkle
trees for tree links and ENRs. The EIP will be updated to match p2p/dnsdisc.

To maintain DNS trees, cmd/devp2p provides a frontend for the p2p/dnsdisc
library. The new 'dns' subcommands can be used to create, sign and deploy DNS
discovery trees.

vendor/github.com/cloudflare/cloudflare-go/duration.go

@@ -0,0 +1,40 @@
+package cloudflare
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// Duration implements json.Marshaler and json.Unmarshaler for time.Duration
+// using the fmt.Stringer interface of time.Duration and time.ParseDuration.
+type Duration struct {
+	time.Duration
+}
+
+// MarshalJSON encodes a Duration as a JSON string formatted using String.
+func (d Duration) MarshalJSON() ([]byte, error) {
+	return json.Marshal(d.Duration.String())
+}
+
+// UnmarshalJSON decodes a Duration from a JSON string parsed using time.ParseDuration.
+func (d *Duration) UnmarshalJSON(buf []byte) error {
+	var str string
+
+	err := json.Unmarshal(buf, &str)
+	if err != nil {
+		return err
+	}
+
+	dur, err := time.ParseDuration(str)
+	if err != nil {
+		return err
+	}
+
+	d.Duration = dur
+	return nil
+}
+
+var (
+	_ = json.Marshaler((*Duration)(nil))
+	_ = json.Unmarshaler((*Duration)(nil))
+)