p2p: support configuring NAT in TOML file

[fjl]

This is an alternative for #27407 with a solution based on gencodec. With the PR, one can now configure like this:

# config.toml
[Node.P2P]
NAT = "extip:33.33.33.33"

$ geth --config config.toml
...
INFO [01-17|16:37:31.436] Started P2P networking      self=enode://[email protected]:30303

[georgehao]

Suggested change

	// Static nodes are used as pre-configured connections which are always
	// StaticNodes are used as pre-configured connections which are always

[georgehao]

Suggested change

	// Connectivity can be restricted to certain IP networks.
	// NetRestrict is used to restrict connectivity to specific IP networks.

[georgehao]

Suggested change

	// If ListenAddr is set to a non-nil address, the server
	// will listen for incoming connections.
	// ListenAddr specifies the address on which the server will listen for incoming connections.
	// If set to a non-empty value, the server will actively listen for connections on this address.

[georgehao]

Suggested change

	// If DiscAddr is set to a non-nil value, the server will use ListenAddr
	// for TCP and DiscAddr for the UDP discovery protocol.
	// DiscAddr specifies the address to be used for the UDP discovery protocol.
	// If set to a non-empty value, the server will use ListenAddr for TCP connections
	// and DiscAddr for UDP-based discovery communication.

[georgehao]

Suggested change

	// If set to a non-nil value, the given NAT port mapper
	// is used to make the listening port available to the
	// Internet.
	// NAT specifies the NAT port mapping mechanism to use for making the listening port
	// accessible from the Internet. If set to a non-nil value, the provided NAT interface
	// will be used to map and expose the server's listening port.

[georgehao]

Suggested change

	// If Dialer is set to a non-nil value, the given Dialer
	// is used to dial outbound peer connections.
	// Dialer specifies the dialer used to establish outbound peer connections.
	// If set to a non-nil value, the provided dialer will be used to dial and connect
	// to remote peers. If nil, a default dialing mechanism will be used.

[georgehao]

Suggested change

	// If NoDial is true, the server will not dial any peers.
	// NoDial controls whether the server will initiate outbound connections to peers.
	// If set to true, the server will not dial any peers and will only accept incoming connections.

[georgehao]

Suggested change

	// If EnableMsgEvents is set then the server will emit PeerEvents
	// whenever a message is sent to or received from a peer
	// EnableMsgEvents enables the emission of PeerEvents for message activity.
	// If set to true, the server will emit events whenever a message is sent to or received from a peer.

[georgehao]

Suggested change

	// This field must be set to a valid secp256k1 private key.
	// PrivateKey specifies the secp256k1 private key used for cryptographic operations.
	// This field must be set to a valid ECDSA private key; otherwise, the server will not function properly.

[georgehao]

Suggested change

	// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
	// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.