drm-lease-v1: specify withdrawn event for client leasing the connector

The drm-lease-v1 protocol doesn't specify if a client holding the lease should get the withdrawn event, and what to make of it if it receives it while holding a lease for the connector.

    <event name="withdrawn">
      <description summary="lease offer withdrawn">
        Sent to indicate that the compositor will no longer honor requests for
        DRM leases which include this connector. The client may still issue a
        lease request including this connector, but the compositor will send
        wp_drm_lease_v1.finished without issuing a lease fd. Compositors are
        encouraged to send this event when they lose access to connector, for
        example when the connector is hot-unplugged, when the connector gets
        leased to a client or when the compositor loses DRM master.
      </description>
    </event>

We recently received an issue in Monado (monado/monado!2283 (merged)) from a user using the Niri compositor, where it would send a withdrawn event right after leasing the connector.

This behaviour should be specified to help clients know what to do of this.