net/mlx5: Introduce node struct and rename group terminology to node

Introduce the `mlx5_esw_sched_node` struct, consolidating all rate
hierarchy related details, including membership and scheduling
parameters.

Since the group concept aligns with the `mlx5_esw_sched_node`, replace
the `mlx5_esw_rate_group` struct with it and rename the "group"
terminology to "node" throughout the rate hierarchy.

All relevant code paths and structures have been updated to use the
"node" terminology accordingly, laying the groundwork for future
patches that will unify the handling of different types of members
within the rate hierarchy.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>