Implement control plane resize

Magnum supports control plane resizing as of spec[1] and patchset[2].

The feature was released in Magnum 2025.1. Prior versions of Magnum
will not call this driver function, but remain safe to use with this
change.

[1] https://review.opendev.org/c/openstack/magnum-specs/+/905281
[2] https://review.opendev.org/c/openstack/magnum/+/906086

Change-Id: I84743c3ead774dd4ab067104e287175386ff60fa

magnum_capi_helm/driver.py

@@ -71,6 +71,11 @@ class Driver(driver.Driver):
             },
         ]
 
+    def validate_master_resize(self, node_count):
+        # This driver supports resizing to the same values
+        # as initial create, so re-use the Base class validation.
+        return self.validate_master_size(node_count)
+
     def _update_control_plane_nodegroup_status(self, cluster, nodegroup):
         # The status of the master nodegroup is determined by the Cluster API
         # control plane object

releasenotes/notes/control-plane-resize-19cc0926a0ab92f9.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Adds support for resizing control plane nodes. This feature will be
+    possible only when using Magnum 2025.1 or above, and will otherwise
+    provide no changes.
+    Usage with command line client: openstack coe cluster resize
+    --nodegroup default-master <cluster> <new size>