Add trunk subports support for OpenStackMachine ports

[bnallapeta]

/kind feature

Describe the solution you'd like
Currently, CAPO supports creating trunk ports for OpenStackMachine resources, but it does not provide a way to specify subports (e.g., VLANs) to be attached to those trunks. Users who need to configure complex networking topologies involving trunked VLANs cannot do so directly through the CAPO API.

This feature adds the ability to define subports within the ports configuration of OpenStackMachine.

Anything else you would like to add:

Example usage:

apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
kind: OpenStackMachineTemplate
spec:
  template:
    spec:
      ports:
        - network:
            filter:
              name: trunk-network
          trunk: true
          subports:
            - segmentationID: 101
              segmentationType: vlan
              network:
                filter:
                  name: vlan-101-network
              fixedIPs:
                - subnet:
                    filter:
                      name: vlan-101-subnet