feat: implement FindNode message encoding (#109)

**Motivation**

Having some functionality of the discv4 protocol

**Description**

Enables `FindNode` message encoding

Closes #82

crates/net/src/discv4.rs

@@ -5,7 +5,7 @@ use ethrex_core::rlp::{
     error::RLPDecodeError,
     structs::{self, Decoder, Encoder},
 };
-use ethrex_core::H256;
+use ethrex_core::{H256, H512};
 use k256::ecdsa::{signature::Signer, SigningKey};
 use std::net::IpAddr;
 
@@ -18,7 +18,7 @@ pub(crate) enum Message {
     /// A ping message. Should be responded to with a Pong message.
     Ping(PingMessage),
     Pong(PongMessage),
-    FindNode(()),
+    FindNode(FindNodeMessage),
     Neighbors(()),
     ENRRequest(()),
     ENRResponse(()),
@@ -33,6 +33,7 @@ impl Message {
         match self {
             Message::Ping(msg) => msg.encode(&mut data),
             Message::Pong(msg) => msg.encode(&mut data),
+            Message::FindNode(msg) => msg.encode(&mut data),
             _ => todo!(),
         }
 
@@ -161,6 +162,31 @@ impl RLPEncode for PingMessage {
     }
 }
 
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) struct FindNodeMessage {
+    /// The target is a 64-byte secp256k1 public key.
+    target: H512,
+    /// The expiration time of the message. If the message is older than this time,
+    /// it shouldn't be responded to.
+    expiration: u64,
+}
+
+impl FindNodeMessage {
+    #[allow(unused)]
+    pub fn new(target: H512, expiration: u64) -> Self {
+        Self { target, expiration }
+    }
+}
+
+impl RLPEncode for FindNodeMessage {
+    fn encode(&self, buf: &mut dyn BufMut) {
+        structs::Encoder::new(buf)
+            .encode_field(&self.target)
+            .encode_field(&self.expiration)
+            .finish();
+    }
+}
+
 impl RLPDecode for PingMessage {
     fn decode_unfinished(rlp: &[u8]) -> Result<(Self, &[u8]), RLPDecodeError> {
         let decoder = Decoder::new(rlp)?;
@@ -358,6 +384,30 @@ mod tests {
         assert_eq!(result, expected);
     }
 
+    #[test]
+    fn test_encode_find_node_message() {
+        let target: H512 = H512::from_str("d860a01f9722d78051619d1e2351aba3f43f943f6f00718d1b9baa4101932a1f5011f16bb2b1bb35db20d6fe28fa0bf09636d26a87d31de9ec6203eeedb1f666").unwrap();
+        let expiration: u64 = 17195043770;
+
+        let msg = Message::FindNode(FindNodeMessage::new(target, expiration));
+
+        let key_bytes =
+            H256::from_str("577d8278cc7748fad214b5378669b420f8221afb45ce930b7f22da49cbc545f3")
+                .unwrap();
+        let signer = SigningKey::from_slice(key_bytes.as_bytes()).unwrap();
+
+        let mut buf = Vec::new();
+
+        msg.encode_with_header(&mut buf, signer);
+        let result = to_hex(&buf);
+        let hash = "dabe1b1a4dc26324120594091bb94dbdd4aa98326cbfecb60a4a67778ebb0e67";
+        let signature = "28cf71e9a929fa29f4e528f5fdc96e25a3086a777c8967710ddc1ef5f456bae40808baf0840d0449ea5a17ba11bbb012719e679cf3cf8d137106884c393ef15b00";
+        let pkt_type = "03";
+        let encoded_message = "f848b840d860a01f9722d78051619d1e2351aba3f43f943f6f00718d1b9baa4101932a1f5011f16bb2b1bb35db20d6fe28fa0bf09636d26a87d31de9ec6203eeedb1f666850400e78bba";
+        let expected = [hash, signature, pkt_type, encoded_message].concat();
+        assert_eq!(result, expected);
+    }
+
     #[test]
     fn test_decode_pong_message_with_enr_seq() {
         let hash = "2e1fc2a02ad95a1742f6dd41fb7cbff1e08548ba87f63a72221e44026ab1c347";