Add new IC2 actions to stateful test suite

[dcherian]

For https://github.com/earth-mover/icechunk/blob/main/icechunk-python/tests/test_stateful_repo_ops.py

• add a V1 -> V2 migration step
• add amend

For https://github.com/earth-mover/icechunk/blob/main/icechunk-python/tests/test_zarr/test_stateful.py

• add shift_array
• add insert