UCG needs address lookup callbacks

[alex--m]

For reference, UCG parameters require the following:

typedef struct ucg_params {

    /* Callback functions for address lookup, used at connection establishment */
    struct {
        int (*lookup_f)(void *cb_group_context,
                        ucg_group_member_index_t index,
                        ucp_address_t **addr,
                        size_t *addr_len);
        void (*release_f)(ucp_address_t *addr);
    } address;

Currently, the OMPI-based implementation satisfies this requirement as follows:

int mca_coll_ucx_resolve_address(void *cb_group_obj,
                                 ucg_group_member_index_t rank,
                                 ucp_address_t **addr,
                                 size_t *addr_len)
{
    /* Sanity checks */
    ompi_communicator_t* comm = (ompi_communicator_t*)cb_group_obj;
    if (rank == (ucg_group_member_index_t)comm->c_my_rank) {
        COLL_UCX_ERROR("mca_coll_ucx_resolve_address(rank=%lu)"
                       "shouldn't be called on its own rank (loopback)", rank);
        return 1;
    }

    /* Check the cache for a previously established connection to that rank */
    ompi_proc_t *proc_peer =
          (struct ompi_proc_t*)ompi_comm_peer_lookup((ompi_communicator_t*)cb_group_obj, rank);
    *addr = proc_peer->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_COLL];
    *addr_len = 0; /* UCX doesn't need the length to unpack the address */
    if (*addr) {
       return 0;
    }

    /* Obtain the UCP address of the remote */
    int ret = mca_coll_ucx_recv_worker_address(proc_peer, addr, addr_len);
    if (ret < 0) {
        COLL_UCX_ERROR("mca_coll_ucx_recv_worker_address(proc=%d rank=%lu) failed",
                       proc_peer->super.proc_name.vpid, rank);
        return 1;
    }

    /* Cache the connection for future invocations with this rank */
    proc_peer->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_COLL] = *addr;
    return 0;
}

void mca_coll_ucx_release_address(ucp_address_t *addr)
{
    /* no need to free - the address is stored in proc_peer->proc_endpoints */
}

[alex--m]

Currently using ucc_team_p2p_conn, should satisfy this requirement.