p2p/discover: add initial discovery v5 implementation (#20750)

This adds an implementation of the current discovery v5 spec.

There is full integration with cmd/devp2p and enode.Iterator in this
version. In theory we could enable the new protocol as a replacement of
discovery v4 at any time. In practice, there will likely be a few more
changes to the spec and implementation before this can happen.

p2p/discover/common.go

@@ -20,8 +20,10 @@ import (
 	"crypto/ecdsa"
 	"net"
 
+	"github.com/ethereum/go-ethereum/common/mclock"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/p2p/netutil"
 )
 
@@ -39,10 +41,25 @@ type Config struct {
 	PrivateKey *ecdsa.PrivateKey
 
 	// These settings are optional:
-	NetRestrict *netutil.Netlist  // network whitelist
-	Bootnodes   []*enode.Node     // list of bootstrap nodes
-	Unhandled   chan<- ReadPacket // unhandled packets are sent on this channel
-	Log         log.Logger        // if set, log messages go here
+	NetRestrict  *netutil.Netlist   // network whitelist
+	Bootnodes    []*enode.Node      // list of bootstrap nodes
+	Unhandled    chan<- ReadPacket  // unhandled packets are sent on this channel
+	Log          log.Logger         // if set, log messages go here
+	ValidSchemes enr.IdentityScheme // allowed identity schemes
+	Clock        mclock.Clock
+}
+
+func (cfg Config) withDefaults() Config {
+	if cfg.Log == nil {
+		cfg.Log = log.Root()
+	}
+	if cfg.ValidSchemes == nil {
+		cfg.ValidSchemes = enode.ValidSchemes
+	}
+	if cfg.Clock == nil {
+		cfg.Clock = mclock.System{}
+	}
+	return cfg
 }
 
 // ListenUDP starts listening for discovery packets on the given UDP socket.
@@ -51,8 +68,15 @@ func ListenUDP(c UDPConn, ln *enode.LocalNode, cfg Config) (*UDPv4, error) {
 }
 
 // ReadPacket is a packet that couldn't be handled. Those packets are sent to the unhandled
-// channel if configured. This is exported for internal use, do not use this type.
+// channel if configured.
 type ReadPacket struct {
 	Data []byte
 	Addr *net.UDPAddr
 }
+
+func min(x, y int) int {
+	if x > y {
+		return y
+	}
+	return x
+}