Subnet & SubnetManager interface added

interface/network/Subnet.py

+import json
+
+
+class Subnet:
+    def __init__(self,
+                 id,
+                 network_id,
+                 name,
+                 cidr,
+                 ip_version,
+                 gateway_ip,
+                 is_dhcp_enabled,
+                 created_at
+                 ) -> None:
+        super().__init__()
+
+        self.id = id
+        self.network_id = network_id
+        self.name = name
+        self.cidr = cidr
+        self.ip_version = ip_version
+        self.gateway_ip = gateway_ip
+        self.is_dhcp_enabled = is_dhcp_enabled
+        self.created_at = created_at
+
+    def __str__(self) -> str:
+        return self.toJSON()
+
+    def toJSON(self):
+        return json.dumps(self.__dict__)

interface/network/SubnetManager.py

+from interface.network.Subnet import Subnet
+
+
+class SubnetManager:
+    def __init__(self) -> None:
+        super().__init__()
+
+    def create(self, network_id, ip_version, cidr) -> Subnet:
+        raise NotImplementedError
+
+    def get(self, id) -> Subnet:
+        raise NotImplementedError
+
+    def delete(self, id) -> bool:
+        raise NotImplementedError
+
+    def list(self) -> []:
+        raise NotImplementedError