feat: understack trunk

[mfencik]

This PR will enable us to set "tenant_vlan_id" on UCVNI in Nautobot when first subport is added to neutron trunk and user specifies the "segmentation_id" when adding the subport. The idea is that we will use the "tenant_vlan_id" as mapped VLAN that is local to the switchport during server provisioning.
This PR covers only the trunk driver part and there will be another one where we will adjust the understack mechanism driver, so when server will go through provisioning, we will detect the trunk_details on the port attached and do the needful to configure our network.

[skrobul]

Overall looks good, but have a few minor suggestions for potential improvements (comments inline).
Also the UnderStackTrunkDriver class could really use some docstrings, at least on the public methods with a high level explanation of why we react to certain events and what we intend to do.

Another potential refactoring opportunity is in how the configure_tenant_vlan_id and _subports_added are split, consider something similar to this:

    def _configure_subports(self, subports: list[SubPort]) -> None:
        for subport in subports:
            network_id = utils.fetch_subport_network_id(subport_id=subport.port_id)
            tenant_vid = self.nb.fetch_tenant_vid(
                network_id=network_id
            )

            if tenant_vid:
                self._update_vni(tvid, subport)
            elif tenant_vid != subport.segmentation_id:
                self._handle_mismatch(tenant_vid, subport, network_id)


    def _update_vni(tvid, subport):
        ucvni_uuid = utils.fetch_subport_network_id(subport_id=subport.port_id)
        self.nb.add_tenant_vlan_tag_to_ucvni(
            network_uuid=ucvni_uuid, vlan_tag=subport.segmentation_id
        )
        LOG.info(
            "Segmentation ID: %(seg_id)s is now set on Nautobot's UCVNI "
                "UUID: %(ucvni_uuid)s in the tenant_vlan_id custom field",
            {"seg_id": subport.segmentation_id, "ucvni_uuid": ucvni_uuid},
        )

    def _handle_mismatch(tenant_vlan_id, subport, subport_network_id):
        if tenant_vlan_id != subport.segmentation_id:
            err = SubportSegmentationIDError(
                seg_id=subport.segmentation_id,
                net_id=subport_network_id,
                nb_seg_id=tenant_vlan_id,
                subport_id=subport.port_id,
            )

            subport.delete()
            raise err

^ untested