Add group-add for podman pod create

[ikim27]

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Does this PR introduce a user-facing change?

Added the `--group-add` flag to `podman pod create` to allow specifying supplementary groups for the infra container.

[mheon]

I think we'd prefer not to add this; it doesn't make any sense, honestly, as it only affects the infra container, which is never running things anyways. For the one use-case where this would fix things, we'd prefer to change to the newer kernel mount API instead, which would not require an otherwise-unused user-facing flag.