Implemented Heartbeat/Checkin request with agentId/listenerId in request body to simplify listener URLs

src/agents/monarch/core/heartbeat.nim

@@ -0,0 +1,48 @@
+import times 
+
+import ../../../common/[types, serialize, utils]
+
+proc createHeartbeat*(config: AgentConfig): Heartbeat = 
+    return Heartbeat(
+        header: Header(
+            magic: MAGIC,
+            version: VERSION, 
+            packetType: cast[uint8](MSG_HEARTBEAT),
+            flags: cast[uint16](FLAG_PLAINTEXT),
+            seqNr: 0'u32, # Sequence number is not used for heartbeats
+            size: 0'u32,
+            hmac: default(array[16, byte])
+        ), 
+        agentId: uuidToUint32(config.agentId),
+        listenerId: uuidToUint32(config.listenerId),
+        timestamp: uint32(now().toTime().toUnix())
+    )
+
+proc serializeHeartbeat*(request: Heartbeat): seq[byte] =
+
+    var packer = initPacker()
+
+    # Serialize check-in / heartbeat request
+    packer 
+        .add(request.agentId)
+        .add(request.listenerId)
+        .add(request.timestamp)
+
+    let body = packer.pack()
+    packer.reset()
+
+    # TODO: Encrypt check-in / heartbeat request body 
+
+    # Serialize header
+    packer
+        .add(request.header.magic)
+        .add(request.header.version)
+        .add(request.header.packetType)
+        .add(request.header.flags)
+        .add(request.header.seqNr) 
+        .add(cast[uint32](body.len))
+        .addData(request.header.hmac)
+
+    let header = packer.pack()
+
+    return header & body