Add step to Cube rolling windows

[Kkkakania]

Description

Adds a step argument to Cube.rolling_window, matching the strided-window support already provided by iris.util.rolling_window. The default remains step=1, so existing calls keep their current behavior.

The step is applied consistently to cube shape selection, data windows, and numeric/string coordinates. Invalid non-positive steps now raise the same clear error as the utility function.

Fixes #7217.

Checklist

• Included a What's New entry
• Incorporated type hints in changed code
• Added tests for data, dataless cubes, coordinates, and validation
• Checked that no benchmark update is needed
• Updated the public method docstring
• Checked that no dependency update is needed

Validation

• pytest lib/iris/tests/unit/cube/test_Cube.py -q (496 passed, 34 skipped)
• pytest lib/iris/tests/test_analysis.py::TestRollingWindow -q (9 passed)
• pre-commit run --files lib/iris/cube.py lib/iris/tests/unit/cube/test_Cube.py changelog/7232.feature.rst

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