Add UDN layer3 class (#2184)

* Add UDN layer3 class

* Move constants from global scope to class scope

* Remove redundant function call

* Fix key error

* Add missing value argument validation

* Use dictionary access instead of class attributes

* Make parameter optional in a method

* Correct the docstring for the role parameter

* Improve error message for missing required arguments

* Refactor to follow conventions

* Revert "Refactor to follow conventions"

This reverts commit 5d4b942.

* Refactor type annotation

* Refactor condition statement

* Refactor docstring for layer3 subnets

ocp_resources/user_defined_network.py

@@ -128,3 +128,64 @@ def to_dict(self) -> None:
 
             if self.ipam_lifecycle:
                 _layer2["ipamLifecycle"] = self.ipam_lifecycle
+
+
+class Layer3UserDefinedNetwork(UserDefinedNetwork):
+    """
+    UserDefinedNetwork layer3 object.
+
+    API reference:
+    https://ovn-kubernetes.io/api-reference/userdefinednetwork-api-spec/#layer3config
+    """
+
+    LAYER3: str = "Layer3"
+
+    def __init__(
+        self,
+        role: Optional[str] = None,
+        mtu: Optional[int] = None,
+        subnets: Optional[List[Dict[str, Any]]] = None,
+        join_subnets: Optional[List[str]] = None,
+        **kwargs,
+    ):
+        """
+        Create and manage UserDefinedNetwork with layer3 configuration
+
+        Args:
+            role (Optional[str]): role describes the network role in the pod.
+            mtu (Optional[int]): mtu is the maximum transmission unit for a network.
+            subnets (Optional[List[Dict]]): subnets are used for the pod network across the cluster, each expecting:
+                - `cidr` (str): IP range in CIDR notation.
+                - `hostSubnet` (Optional[int]): Host-specific subnet.
+                API reference:
+                https://ovn-kubernetes.io/api-reference/userdefinednetwork-api-spec/#layer3subnet
+            join_subnets (Optional[List[str]]): join_subnets are used inside the OVN network topology.
+        """
+        super().__init__(
+            topology=self.LAYER3,
+            **kwargs,
+        )
+        self.role = role
+        self.mtu = mtu
+        self.subnets = subnets
+        self.join_subnets = join_subnets
+
+    def to_dict(self) -> None:
+        super().to_dict()
+        if not self.kind_dict and not self.yaml_file:
+            if not self.role:
+                raise MissingRequiredArgumentError(argument="role")
+            if not self.subnets:
+                raise MissingRequiredArgumentError(argument="subnets")
+
+            self.res["spec"][self.LAYER3.lower()] = {"role": self.role}
+            _layer3 = self.res["spec"][self.LAYER3.lower()]
+
+            if self.mtu:
+                _layer3["mtu"] = self.mtu
+
+            if self.join_subnets:
+                _layer3["joinSubnets"] = self.join_subnets
+
+            if self.subnets:
+                _layer3["subnets"] = self.subnets