Allow subport creation when using Neutron Trunks

[pbasov]

/kind feature

Describe the solution you'd like
Right now CAPO allows you to create a trunk from the parent port, by setting trunk: true in PortOpts spec.
This however is a bit useless by itself, since there's no capability to create the trunk.

I would like to introduce a new key called subports with the same filter logic as in ports.network that would allow you to create trunk subports in addition to the parent-port + trunk pair.

      ports:
      - network:
          filter:
            name: foo_vlan_300
        trunk: true
        subports:
        - network:
            filter:
              name: bar_vlan_301
          segmentationID: 301
          segmentationType: "vlan"

Anything else you would like to add:
Upstream gophercloud supports it: https://github.com/gophercloud/gophercloud/blob/v1.14.1/openstack/networking/v2/extensions/trunks/requests.go#L162

E2E tests in CAPO already exist: https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/main/test/e2e/suites/e2e/e2e_test.go#L587-L588