Adding a manager based RL env for Franka cube stacking task. This
environment will be used for initial testing of MimicGen pipeline. The
environment consists of a Franka arm with three blocks on a table top.

Environment source is added to
`omni/isaac/lab_tasks/manager_based/manipulation/stack`

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Signed-off-by: peterd-NV <peterd@nvidia.com>
Co-authored-by: CY Chen <cyc@nvidia.com>